diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 8aa8986..12c939e 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -35,13 +35,15 @@ pnpm infrastructure:config
   private source paths.
 - Use `workspace:` ranges for internal package dependencies.
 - Modules own their own UI and author it in React and TypeScript (ADR 0022). A module package may
-  contain `.tsx` and depend on `react`, `react-dom`, and `@ekklesia/module-ui-sdk`. Every other UI
-  framework, `.jsx`, stylesheets, and other presentation formats remain rejected by
-  `pnpm boundaries`. Primary and display sources live at the exact `src/ui/index.tsx` and
+  contain `.tsx` and depend on `react`, `react-dom`, and `@ekklesia/module-ui-sdk`, and may author
+  plain `.css` inside a browser-artifact source root (ADR 0040). Every other UI framework, `.jsx`,
+  other stylesheet dialects, and other presentation formats remain rejected by `pnpm boundaries`. Primary and display sources live at the exact `src/ui/index.tsx` and
   `src/display/index.tsx` conventions; one `pnpm build` invocation atomically compiles every role
   the v6 manifest declares via `@ekklesia/module-ui-build` (ADR 0023/0033). Module authors write no
-  bundler configuration, and styling is React inline style objects only for now. Both roles use the
-  same UI SDK context with no presentation-mode flag.
+  bundler configuration. A module's stylesheets are compiled into its declared artifact as text and
+  installed by the host into that artifact's own Shadow DOM root, so module styles cannot leak into
+  the Dashboard or another module (ADR 0040). Both roles use the same UI SDK context with no
+  presentation-mode flag, and the same versioned mount ABI.
 - A module may also consume exactly two shared design packages — `@ekklesia/design-tokens` and
   `@ekklesia/ui` (ADR 0039) — so two modules' screens look like one product. That is a named
   allowlist: every other shared role (`api-client`, `auth-client`, `surface-sdk`, `module-ui-host`)
diff --git a/apps/api/package.json b/apps/api/package.json
index 2932788..5b50008 100644
--- a/apps/api/package.json
+++ b/apps/api/package.json
@@ -28,11 +28,13 @@
   "devDependencies": {
     "@biomejs/biome": "2.5.3",
     "@ekklesia/module-ui-host": "workspace:*",
+    "@ekklesia/module-ui-sdk": "workspace:*",
     "@ekklesia/typescript-config": "workspace:*",
     "@types/node": "24.13.3",
     "@types/pg": "8.20.0",
     "@types/smtp-server": "3.5.13",
     "better-auth": "1.6.23",
+    "happy-dom": "20.0.11",
     "pg": "8.22.0",
     "smtp-server": "3.19.3",
     "typescript": "7.0.2",
diff --git a/apps/api/tests/module-ui-host-e2e.test.ts b/apps/api/tests/module-ui-host-e2e.test.ts
index cc857b4..2fdb391 100644
--- a/apps/api/tests/module-ui-host-e2e.test.ts
+++ b/apps/api/tests/module-ui-host-e2e.test.ts
@@ -20,7 +20,13 @@ import {
   discoverModules,
   startDiscoveredModuleRuntime,
 } from "@ekklesia/core/modules";
-import { loadModuleUi } from "@ekklesia/module-ui-host";
+import {
+  loadModuleUi,
+  MODULE_UI_ISOLATION_ATTRIBUTE,
+  MODULE_UI_ISOLATION_ROOT_ATTRIBUTE,
+} from "@ekklesia/module-ui-host";
+import { MODULE_UI_ABI_VERSION } from "@ekklesia/module-ui-sdk";
+import { Window } from "happy-dom";
 import {
   bootstrapDisplayHost,
   bootstrapHost,
@@ -165,6 +171,7 @@ async function createFixtureModule(
   await writeFile(
     path.join(moduleDirectory, "dist/ui/index.js"),
     `export default {
+  abi: ${MODULE_UI_ABI_VERSION},
   id: ${JSON.stringify(id)},
   version: "0.1.0",
   async mount(container, context) {
@@ -184,6 +191,7 @@ async function createFixtureModule(
     await writeFile(
       path.join(moduleDirectory, "dist/display/index.js"),
       `export default {
+  abi: ${MODULE_UI_ABI_VERSION},
   id: ${JSON.stringify(id)},
   version: "0.1.0",
   async mount(container, context) {
@@ -310,10 +318,35 @@ async function publishedArtifactUrl(harness: Harness, sourceUrl: string): Promis
   return `data:text/javascript;base64,${bytes.toString("base64")}`;
 }
 
-/** The loader hands the container straight to the module and never inspects it, so a plain sink is
- * enough and no DOM implementation is needed in this Node-environment suite. */
-function containerSink(): { textContent: string } {
-  return { textContent: "" };
+/**
+ * A real host-owned container, built from `happy-dom` **as a library** rather than as this suite's
+ * test environment.
+ *
+ * Since ADR 0040 the generic loader builds an isolated Shadow DOM root inside whatever container it
+ * is given, so a plain object sink is no longer a faithful stand-in — and the isolation is exactly
+ * what an end-to-end proof should exercise rather than stub out. Installing happy-dom as the
+ * *environment* is not an option here: it replaces the module runner's resolution, and this suite
+ * dynamically imports real module server entrypoints from temporary directories, which then fail to
+ * resolve. Constructing one `Window` gives the DOM these few assertions need and leaves every global
+ * this server-side suite depends on exactly as Node provides it.
+ */
+function containerSink(): HostContainer {
+  return new Window().document.createElement("div") as unknown as HostContainer;
+}
+
+interface HostContainer {
+  readonly children: { readonly length: number };
+  querySelector(selectors: string): HostContainer | null;
+  readonly shadowRoot: { querySelector(selectors: string): { textContent: string } | null } | null;
+}
+
+/** What the mounted artifact rendered, read through its open isolated root. */
+function mountedText(container: HostContainer): string {
+  const shadowHost = container.querySelector(`[${MODULE_UI_ISOLATION_ATTRIBUTE}]`);
+  return (
+    shadowHost?.shadowRoot?.querySelector(`[${MODULE_UI_ISOLATION_ROOT_ATTRIBUTE}]`)?.textContent ??
+    ""
+  );
 }
 
 type LoaderContainer = Parameters<typeof loadModuleUi>[0]["container"];
@@ -365,13 +398,13 @@ describe("end to end: a discovered module's UI mounts in the Ekklesia host", ()
       });
 
       // 5. The module rendered, having reached its own backend through the host.
-      expect(container.textContent).toBe("example:200:2");
+      expect(mountedText(container)).toBe("example:200:2");
       // 6. It asked for "records"; the host resolved that onto Ekklesia's module mount prefix.
       expect(harness.moduleApiCalls).toEqual(["GET /api/v1/modules/example/records"]);
 
       // 7. And it tears down cleanly.
       await mounted.unmount();
-      expect(container.textContent).toBe("");
+      expect(mountedText(container)).toBe("");
       expect(mounted.mounted).toBe(false);
     } finally {
       await harness.stop();
@@ -443,10 +476,10 @@ describe("end to end: a discovered module's UI mounts in the Ekklesia host", ()
         },
       });
 
-      expect(container.textContent).toBe("example:display:200:2");
+      expect(mountedText(container)).toBe("example:display:200:2");
       expect(harness.moduleApiCalls).toEqual(["GET /api/v1/modules/example/records"]);
       await mounted.unmount();
-      expect(container.textContent).toBe("");
+      expect(mountedText(container)).toBe("");
       expect(mounted.mounted).toBe(false);
     } finally {
       await harness.stop();
@@ -514,7 +547,7 @@ describe("end to end: a discovered module's UI mounts in the Ekklesia host", ()
       });
 
       // Hiding a control is a usability improvement: the module simply did not ask.
-      expect(container.textContent).toBe("example:denied");
+      expect(mountedText(container)).toBe("example:denied");
       expect(harness.moduleApiCalls).toEqual([]);
     } finally {
       await harness.stop();
@@ -593,7 +626,7 @@ describe("end to end: a discovered module's UI mounts in the Ekklesia host", ()
           version: groupedEntry.descriptor.version,
         },
       });
-      expect(container.textContent).toBe("example:200:2");
+      expect(mountedText(container)).toBe("example:200:2");
       await mounted.unmount();
 
       // Now move it somewhere else entirely.
@@ -645,7 +678,7 @@ describe("end to end: a discovered module's UI mounts in the Ekklesia host", ()
           version: movedEntry.descriptor.version,
         },
       });
-      expect(movedContainer.textContent).toBe("example:200:2");
+      expect(mountedText(movedContainer)).toBe("example:200:2");
       expect(harness.moduleApiCalls.slice(callsBeforeRemount)).toEqual([
         "GET /api/v1/modules/example/records",
       ]);
diff --git a/apps/api/tests/module-ui-routes.test.ts b/apps/api/tests/module-ui-routes.test.ts
index a59dd9a..7b521dc 100644
--- a/apps/api/tests/module-ui-routes.test.ts
+++ b/apps/api/tests/module-ui-routes.test.ts
@@ -16,6 +16,8 @@ import {
   EMPTY_MODULE_REALTIME_REGISTRY,
 } from "@ekklesia/core/modules";
 import { loadModuleUi } from "@ekklesia/module-ui-host";
+import { MODULE_UI_ABI_VERSION } from "@ekklesia/module-ui-sdk";
+import { Window } from "happy-dom";
 import type { Hono } from "hono";
 import { afterEach, describe, expect, it, vi } from "vitest";
 import { createInMemoryDashboardStore } from "./support/in-memory-dashboard-store.js";
@@ -59,6 +61,7 @@ async function createModulesDirectory(): Promise<string> {
  * sequence whose exact publication and loader-compatibility can be asserted. */
 function artifactSource(id: string, version: string): string {
   return `export default {
+  abi: ${MODULE_UI_ABI_VERSION},
   id: ${JSON.stringify(id)},
   version: ${JSON.stringify(version)},
   mount(container, context) {
@@ -862,6 +865,12 @@ describe("module UI artifact URL generation", () => {
   });
 });
 
+interface HostContainer {
+  readonly children: { readonly length: number };
+  querySelector(selectors: string): HostContainer | null;
+  readonly shadowRoot: { querySelector(selectors: string): { textContent: string } | null } | null;
+}
+
 describe("module UI artifact publication — the generic loader consumes what is published", () => {
   /**
    * Node cannot `import()` an `http:` URL, so the two halves of the chain are proven separately and
@@ -880,16 +889,29 @@ describe("module UI artifact publication — the generic loader consumes what is
   }
 
   /**
-   * The loader forwards the container untouched to the module's own `mount` and never inspects it,
-   * so a plain sink is enough and no DOM implementation is required. The container's type is read
-   * back off `loadModuleUi` itself rather than written as `Element`, because `apps/api` compiles
-   * against Node libs only — naming a DOM type here would be the same browser/server confusion this
-   * route exists to keep apart.
+   * A real host-owned container, because since ADR 0040 the loader builds an isolated Shadow DOM
+   * root inside whatever it is given. The container's type is read back off `loadModuleUi` itself
+   * rather than written as `Element`, because `apps/api` compiles against Node libs only — naming a
+   * DOM type here would be the same browser/server confusion this route exists to keep apart.
    */
   type LoaderContainer = Parameters<typeof loadModuleUi>[0]["container"];
 
-  function containerSink(): { textContent: string } {
-    return { textContent: "" };
+  /**
+   * `happy-dom` used as a library rather than as this suite's environment: the loader now builds a
+   * real isolated root inside the container, and installing a DOM environment here would replace
+   * the module runner's resolution for a server-side suite that dynamically imports real module
+   * entrypoints.
+   */
+  function containerSink(): HostContainer {
+    return new Window().document.createElement("div") as unknown as HostContainer;
+  }
+
+  /** What the mounted artifact rendered, read through its open isolated root. */
+  function mountedText(container: HostContainer): string {
+    const shadowHost = container.querySelector("[data-ekklesia-module-ui]");
+    return (
+      shadowHost?.shadowRoot?.querySelector("[data-ekklesia-module-ui-root]")?.textContent ?? ""
+    );
   }
 
   function hostContext() {
@@ -915,9 +937,9 @@ describe("module UI artifact publication — the generic loader consumes what is
       source: { moduleId: "announcements", url, version: "1.4.2" },
     });
 
-    expect(container.textContent).toBe("announcements:mounted");
+    expect(mountedText(container)).toBe("announcements:mounted");
     await mounted.unmount();
-    expect(container.textContent).toBe("");
+    expect(container.children).toHaveLength(0);
   });
 
   it("still rejects the published artifact when the host expects a different identity", async () => {
diff --git a/docs/architecture/decisions/0023-module-ui-build-pipeline.md b/docs/architecture/decisions/0023-module-ui-build-pipeline.md
index 1cff059..7bc3939 100644
--- a/docs/architecture/decisions/0023-module-ui-build-pipeline.md
+++ b/docs/architecture/decisions/0023-module-ui-build-pipeline.md
@@ -100,6 +100,15 @@ This is deliberately the smallest viable styling story. A real stylesheet contra
 field, a host loading mechanism, and a style-isolation decision, none of which exist — and inventing
 them here would expand the manifest and runtime contract before a host can validate them.
 
+> **Correction — this rule is superseded.**
+> [ADR 0040](0040-module-stylesheets-and-artifact-isolation.md) supplies the three missing pieces and
+> reverses this one decision: a module may import plain `.css` from a browser-artifact source root,
+> the canonical builder compiles it **into** the declared artifact as text (so no sidecar is ever
+> emitted, which is what this section was protecting), and the host installs it into a Shadow DOM
+> root it creates for every artifact. No manifest field was needed after all. The rest of this ADR —
+> esbuild, bundled React, fixed source paths, no author bundler configuration, unminified output — is
+> unchanged.
+
 ### Browser TypeScript stays scoped to UI source
 
 `@ekklesia/typescript-config/browser-react.json` extends the existing `browser.json` and sets
diff --git a/docs/architecture/decisions/0040-module-stylesheets-and-artifact-isolation.md b/docs/architecture/decisions/0040-module-stylesheets-and-artifact-isolation.md
new file mode 100644
index 0000000..3722420
--- /dev/null
+++ b/docs/architecture/decisions/0040-module-stylesheets-and-artifact-isolation.md
@@ -0,0 +1,166 @@
+# 0040: Module stylesheets, and every browser artifact gets an isolated root
+
+- Status: Accepted
+- Date: 2026-08-27
+- Supersedes: none
+- Related: [ADR 0021](0021-modules-as-hostable-products.md),
+  [ADR 0022](0022-module-ui-authoring-and-host-contract.md),
+  [ADR 0023](0023-module-ui-build-pipeline.md),
+  [ADR 0024](0024-browser-safe-ui-sdk-and-host-loader.md),
+  [ADR 0033](0033-display-kiosk-host-contract.md),
+  [ADR 0039](0039-shared-design-tokens-and-elements.md)
+
+## Context
+
+ADR 0023 rejected stylesheet imports in module UI, and gave an honest reason: esbuild would emit a
+`.css` sidecar beside the artifact, nothing would declare it, no host would fetch it, and it would
+*work in a local test and silently do nothing in a real host*. Inline style objects were the whole
+supported story, and a stylesheet contract was deferred to "a manifest field, a host loading
+mechanism, and a style-isolation decision, none of which exist yet."
+
+Two of those three now exist or are unnecessary, and the remaining gap is blocking real work:
+
+- Inline style objects cannot express `:hover`, `:focus-visible`, `:disabled`, `@media`, or
+  `@keyframes`. A production management screen needs all of them, and an accessible focus treatment
+  needs `:focus-visible` specifically. This is not a preference; it is a capability inline styles do
+  not have.
+- ADR 0039's shared elements are built from real CSS for exactly that reason, so a module using them
+  already needs somewhere for a stylesheet to live.
+- A contributed renderer (a later ADR) makes the isolation question urgent rather than theoretical:
+  one module's renderer mounted inside another module's screen is precisely where a leaked selector
+  does damage nobody can trace.
+
+The manifest field turns out not to be needed, and the loading mechanism and the isolation decision
+turn out to be the same decision.
+
+## Decision
+
+### A stylesheet is compiled **into** the declared artifact, as text
+
+A module imports its local `.css` files. `@ekklesia/module-ui-build` compiles each one — resolving
+local `@import`s through a nested bundle — into a JavaScript module whose default export is that
+stylesheet's text. The author passes the text to `defineModuleUi({ styles })`.
+
+That is why no manifest field is required. There is still exactly **one** declared file per browser
+role, containing everything: nothing new to publish, nothing for a host to fetch, no second URL, and
+no way for a stylesheet to become an undeclared sidecar — the failure ADR 0023 named. The builder
+additionally fails if any CSS output file is ever produced beside an artifact, so a future option or
+loader change cannot quietly reintroduce one.
+
+Two rules are enforced on the compiled text, each with its own sanitized error code:
+
+- **A module stylesheet may reference Ekklesia's global theme variables and must never declare one.**
+  Declaring `--ekklesia-*` would invent a global outside the registry a future theme editor
+  enumerates (ADR 0039). A private alias under the module's own prefix is the supported way to name
+  a local value, and is explicitly allowed.
+- **An `@import` that survives compilation is rejected.** A local import is inlined and leaves
+  nothing behind, so anything remaining is a remote or unresolvable reference, and an artifact that
+  fetches a stylesheet at run time is not self-contained.
+
+Plain `.css` only. `.scss`, `.less`, `.styl`, and friends stay rejected: adding a preprocessor is a
+language and toolchain decision needing its own ADR, not a detail of a build pass.
+
+`pnpm boundaries` allows `.css` inside a module **only** under the browser-artifact source roots the
+canonical builder actually compiles (`src/ui/`, `src/display/`, and the contribution sources a later
+ADR adds). Anywhere else in a module a stylesheet is a file no build reads and no host loads — the
+undeclared sidecar again, just relocated. Every other stylesheet dialect stays rejected everywhere.
+
+### Every browser artifact mounts inside an isolated root
+
+The generic loader creates a Shadow DOM root inside the host's container, installs the artifact's
+stylesheet there, and hands the artifact an element inside that root:
+
+```text
+host container            ← the host's element; the loader never mutates it beyond this one child
+└─ <div data-ekklesia-module-ui>            ← shadow host, created and removed by the loader
+     #shadow-root (open)
+       ├─ <style data-ekklesia-module-ui-styles>  base rules, then the artifact's declared CSS
+       └─ <div data-ekklesia-module-ui-root>      ← the Element handed to mount()
+```
+
+Shadow DOM is chosen because it is the one mechanism in a browser that **enforces** the rule rather
+than describing it: rules inside a shadow root do not apply outside it, and document rules do not
+apply inside it. A class-name convention, a build-time prefix, or a "please scope your selectors"
+instruction are conventions that one mistake defeats, and the failure mode is a module silently
+restyling the Dashboard or another module's renderer.
+
+This applies to **every** role — primary UI, display UI, and contributed renderers — unconditionally,
+including artifacts that declare no stylesheet. One mounting path, not a styled one and an unstyled
+one, because a conditional isolation boundary is one a module can accidentally fall out of.
+
+Both wrapper elements are `display: contents` through a base stylesheet the loader installs *before*
+the artifact's own, so the loader's two elements contribute no boxes and an artifact lays out exactly
+as it did before isolation existed — and an artifact that genuinely needs a box (a full-viewport
+display) overrides `:host` by ordinary cascade order rather than by anything the host special-cases.
+
+`mode: "open"` is deliberate. **This is not a security boundary, and must not be described as one.**
+An installed module is trusted first-party code running in the host's own realm (ADR 0022, ADR 0024);
+it could reach `document` directly if it wanted to. `closed` would buy no protection while making the
+artifact's DOM unreachable to the host's own tests and diagnostics. This is a correctness boundary
+for *styles*.
+
+Design tokens still reach the artifact: CSS custom properties inherit through a shadow boundary, so a
+host that installed the theme at its root (ADR 0039) has already supplied every artifact on the page.
+
+### Mount and unmount are deterministic, in both directions
+
+Each mount builds a fresh root with exactly one `<style>`. Unmount runs the module's cleanup and then
+removes the whole root — content and styles together — in a `finally`, so a module whose cleanup
+throws still has its stylesheet taken out of the page. Remounting cannot accumulate a second copy
+because there is nothing left to accumulate onto. A mount that throws disposes the root it built, so
+a failed artifact leaves neither partial DOM nor installed styles behind.
+
+### The mount ABI is versioned, and the cutover is hard
+
+An artifact's DOM is no longer a direct child of the host's element, and the definition gained a
+field. That is observable, so `MODULE_UI_ABI_VERSION` is now `2` and `defineModuleUi` stamps it —
+the author never writes it, so it can never drift from the SDK the artifact actually compiled
+against.
+
+There is no dual acceptance, exactly as there is none for a manifest version (ADR 0006, ADR 0014,
+ADR 0021, ADR 0033). A version-1 artifact fails with `MODULE_UI_ABI_UNSUPPORTED` — checked first and
+on its own, so the message is "rebuild your module" rather than a developer hunting for a typo.
+Migration is one `pnpm build`: the canonical builder owns every option, and a module that declares no
+stylesheet needs no source change at all.
+
+## Consequences
+
+- Module UI can express hover, focus-visible, disabled, media queries, and keyframes, so ADR 0039's
+  accessible focus treatment actually works inside a module artifact.
+- One module's stylesheet provably cannot reach the Dashboard, another module, or another renderer,
+  and the Dashboard's cannot reach in. Two artifacts of the *same* module are isolated from each
+  other too: they share styles only if they share a source file.
+- A host's own tests reach an artifact's DOM through its open root rather than through
+  `container.querySelector`. Every mounting test in this repository was updated accordingly; that
+  churn is the visible price of the boundary being real.
+- Every module must be rebuilt once. Nothing else changes for an author who uses no stylesheet.
+- The artifact is larger by the size of its CSS, exactly as it is larger by the size of React
+  (ADR 0023). It remains one file.
+- A module still cannot reference a bundled image or font from CSS: no asset loader is configured, so
+  such a reference fails the build. Data URLs work; object storage remains the answer for real media.
+
+## Alternatives
+
+- **Keep inline styles only.** Rejected: it cannot express the pseudo-classes an accessible control
+  needs, and the workaround — JavaScript hover and focus tracking — is worse in every way.
+- **Publish a `.css` sidecar and have the host `<link>` it.** Rejected: it needs a manifest field, a
+  second publication route, a second cache policy, and a second failure mode, and it reintroduces the
+  exact "a file nothing declares" hazard ADR 0023 refused. Compiling into the one declared artifact
+  costs none of that.
+- **Scope styles by build-time class prefixing instead of Shadow DOM.** Rejected: a convention a
+  single unprefixed selector defeats, with a silent failure mode and no way for the host to prove
+  containment. It also cannot stop the *host's* rules from reaching into an artifact.
+- **Isolate only artifacts that declare a stylesheet.** Rejected: an artifact would fall in and out
+  of isolation as a source file changed, and a module rendering the shared elements' `<style>` from
+  inside React would leak into the host document without ever importing a `.css` file.
+- **`mode: "closed"`.** Rejected as security theatre: the artifact is same-realm trusted code, and
+  closing the root only blinds the host's own diagnostics.
+- **Accept both ABI versions.** Rejected for consistency with every manifest cutover this repository
+  has made: dual acceptance means one artifact can be read two ways, and the migration here is a
+  rebuild rather than an edit.
+
+## Deferred
+
+CSS preprocessors, CSS Modules, bundled image and font assets, `adoptedStyleSheets` for shared
+constructable stylesheets across artifacts, a host-supplied reset inside artifacts beyond the two
+layout rules, and any claim that Shadow DOM confines module *code*.
diff --git a/docs/architecture/decisions/README.md b/docs/architecture/decisions/README.md
index 94dede6..70592f0 100644
--- a/docs/architecture/decisions/README.md
+++ b/docs/architecture/decisions/README.md
@@ -48,3 +48,4 @@ ADRs capture durable choices that affect more than one package or phase.
 - [0037: Outbound email is a host-provided transport, and SMTP is its first provider](0037-host-provided-outbound-email.md) — Accepted
 - [0038: Email verification and password recovery, with a trusted-bootstrap exception](0038-email-verification-and-password-recovery.md) — Accepted
 - [0039: Shared design tokens and foundational elements](0039-shared-design-tokens-and-elements.md) — Accepted
+- [0040: Module stylesheets, and every browser artifact gets an isolated root](0040-module-stylesheets-and-artifact-isolation.md) — Accepted
diff --git a/docs/architecture/repository-boundaries.md b/docs/architecture/repository-boundaries.md
index 00f7062..ff0f2ca 100644
--- a/docs/architecture/repository-boundaries.md
+++ b/docs/architecture/repository-boundaries.md
@@ -20,7 +20,11 @@ These rules map platform ownership to filesystem and package boundaries. The exe
 - A module may own UI ([ADR 0021](decisions/0021-modules-as-hostable-products.md)) and authors it in
   React and TypeScript ([ADR 0022](decisions/0022-module-ui-authoring-and-host-contract.md)). A
   module may therefore contain `.tsx` source and depend on `react`, `react-dom`, and
-  `@ekklesia/module-ui-sdk`. It still cannot contain `.jsx`, stylesheets, single-file components,
+  `@ekklesia/module-ui-sdk`. Since
+  [ADR 0040](decisions/0040-module-stylesheets-and-artifact-isolation.md) it may also contain plain
+  `.css`, but **only** inside a browser-artifact source root the canonical builder compiles
+  (`src/ui/`, `src/display/`); a stylesheet anywhere else in a module is a file no build reads and no
+  host loads. It still cannot contain `.jsx`, other stylesheet dialects, single-file components,
   Astro/Marko/MDX/HTML views, or SVG presentation assets, and cannot depend on any other UI framework
   (including `react-router-dom`, `react-native`, and `@vitejs/plugin-react`) or on any shared role
   beyond the two approved design packages named below. A module needs no bundler plugin: `@ekklesia/module-ui-build` — a `tooling` package,
@@ -51,6 +55,14 @@ These rules map platform ownership to filesystem and package boundaries. The exe
   Node's global declarations — into browser code. Core still owns the containing
   application shell and all platform policy, and module UI reaches the host only through
   `@ekklesia/module-ui-sdk`.
+- A module stylesheet is compiled **into** the one declared JavaScript artifact as text and installed
+  by the host at mount; there is no sidecar file, no second publication route, and no manifest field
+  for one. Every browser artifact is mounted inside its own Shadow DOM root the generic loader
+  creates, so one module's styles cannot reach the Dashboard, another module, or another renderer,
+  and the host's cannot reach in. That root is a correctness boundary for styles, never a sandbox:
+  module code remains trusted, same-realm code. The browser mount ABI is versioned
+  (`MODULE_UI_ABI_VERSION`) and a mismatched artifact fails closed rather than mounting under guessed
+  semantics.
 - Manifest v6 permits independently optional primary (`entrypoints.ui`) and full-viewport display
   (`entrypoints.display`) artifacts. Modules own both feature experiences; surfaces own their
   containers, routes, navigation/chrome, credentials, error states, and publication wiring. The
diff --git a/docs/module-development/README.md b/docs/module-development/README.md
index 3e8da61..611ab33 100644
--- a/docs/module-development/README.md
+++ b/docs/module-development/README.md
@@ -471,20 +471,105 @@ Every other shared package stays rejected inside a module: `@ekklesia/api-client
 `@ekklesia/auth-client`, `@ekklesia/surface-sdk`, and `@ekklesia/module-ui-host` are not yours to
 import, and `pnpm boundaries` says so.
 
-### Styling: inline style objects only, for now
+### Styling: a stylesheet compiled into your artifact
 
-Style with React's `style` prop:
+Author plain `.css` beside your UI source, import it, and declare it
+([ADR 0040](../architecture/decisions/0040-module-stylesheets-and-artifact-isolation.md)):
 
-```tsx
-<section style={{ display: "grid", gap: "0.5rem" }}>…</section>
+```text
+src/ui/index.tsx
+src/ui/styles.css
 ```
 
-**Stylesheet imports are not supported, and the build fails on one.** That is deliberate rather than
-an oversight: esbuild would emit a `.css` sidecar beside a primary or display artifact, but no
-manifest field declares it and no host loads it — so it would work in your tests and silently do
-nothing in a real host. A stylesheet contract needs a manifest field, a host loading mechanism, and
-a style-isolation decision, none of which exist yet. Inline styles are the whole supported story for
-this phase.
+```tsx
+import { defineModuleUi } from "@ekklesia/module-ui-sdk";
+import { EKKLESIA_UI_STYLESHEET } from "@ekklesia/ui";
+import styles from "./styles.css";
+
+export default defineModuleUi({
+  id: "announcements",
+  version: "0.1.0",
+  styles: `${EKKLESIA_UI_STYLESHEET}\n${styles}`,
+  mount(container, context) { /* … */ },
+});
+```
+
+TypeScript needs to know what a `.css` import is. Copy the template's `src/module-styles.d.ts` into
+your module and include it in your UI TypeScript project:
+
+```ts
+declare module "*.css" {
+  const stylesheet: string;
+  export default stylesheet;
+}
+```
+
+**There is no sidecar file.** `@ekklesia/module-ui-build` compiles each stylesheet — resolving local
+`@import`s — into your declared JavaScript artifact as text. One declared file still contains
+everything, so there is nothing extra to publish, nothing for a host to fetch, and no way for your
+CSS to become an asset nobody serves. That was the exact hazard that kept stylesheets out until now.
+
+Two rules are enforced by the build, each with its own error:
+
+- **Reference Ekklesia's global theme variables; never declare one.** `--ekklesia-*` belongs to the
+  registry a future theme editor enumerates (ADR 0039). Derive private aliases under your own prefix
+  freely: `--announcements-slide-gap: var(--ekklesia-spacing-sm)`.
+- **Every `@import` must resolve to a local file.** A remote import would make your artifact fetch a
+  stylesheet at run time, and a declared artifact has to be self-contained.
+
+Plain `.css` only. `.scss`, `.less`, and other dialects stay rejected — adding a preprocessor is a
+toolchain decision that needs its own ADR. A CSS `url()` pointing at a local image or font also fails
+the build: no asset loader is configured. Use a data URL, or serve real media from your module's
+object storage.
+
+`pnpm boundaries` allows `.css` inside `src/ui/` and `src/display/` only. A stylesheet anywhere else
+in your module is a file no build reads and no host loads.
+
+### Your artifact renders in its own isolated root
+
+Every browser artifact — primary UI, display UI, and any renderer you contribute — is mounted inside
+a Shadow DOM root the host creates, with your stylesheet installed in it:
+
+```text
+host container
+└─ <div data-ekklesia-module-ui>
+     #shadow-root (open)
+       ├─ <style>  host base rules, then your declared CSS
+       └─ <div>    ← the container your mount() receives
+```
+
+What that buys you, and what it costs:
+
+- **Your selectors cannot leak out**, and the Dashboard's cannot leak in. Write `.slide` without a
+  prefix and without worrying about another module's `.slide`.
+- **Design tokens still reach you.** CSS custom properties inherit through the boundary, so
+  `var(--ekklesia-…)` resolves from the values the host installed at its root. Nothing to import at
+  run time, nothing for the host to pass.
+- **Two artifacts of your own module are isolated from each other too.** Your management UI and your
+  display share styles only if they share a source file.
+- **Your DOM is not in the host document's tree.** `document.querySelector` from outside will not
+  find your elements — including in your own tests, which should mount the artifact and query inside
+  the root the loader hands `mount`.
+- **You get no box by default.** The host's two wrapper elements are `display: contents`, so you lay
+  out exactly where the host put you. A full-viewport display that needs a real box overrides it:
+  `:host { display: block }`, which works because the host installs its base rules *before* yours.
+- **Mount and unmount are deterministic.** The host installs your stylesheet once per mount and
+  removes the whole root on unmount, so repeated mounts never accumulate styles and a cleanup that
+  throws still cannot leave your CSS behind. You never install or remove your own `<style>`.
+
+**This is not a sandbox.** Your artifact is trusted first-party code in the host's own JavaScript
+realm and can reach `document` if it wants to; the root is `open` for exactly that reason. It is a
+correctness boundary for styles, and nothing more. See
+[Security model and deployment trust boundaries](../development/security.md).
+
+### The mount ABI is versioned
+
+`defineModuleUi` stamps the browser mount ABI version onto your artifact — you never write it. A host
+verifies it before mounting, and an artifact built against an older SDK fails with
+`MODULE_UI_ABI_UNSUPPORTED` rather than being mounted under guessed semantics.
+
+Migrating from ABI 1 is one `pnpm build`. If you declare no stylesheet, no source change is needed at
+all; if your own tests assert on module DOM, they need to look inside the isolated root.
 
 ### What you may and may not use today
 
@@ -493,7 +578,7 @@ the build pipeline** — adding the pipeline required no boundary relaxation at
 
 | Allowed in a module | Still rejected |
 | --- | --- |
-| `.tsx` source | `.jsx` (strict TypeScript only), `.css`, `.vue`, `.svelte`, `.astro`, `.mdx`, `.svg`, `.html` |
+| `.tsx` source, and plain `.css` inside `src/ui/` or `src/display/` | `.jsx` (strict TypeScript only), `.css` outside a browser source root, `.scss`/`.less`/`.styl`, `.vue`, `.svelte`, `.astro`, `.mdx`, `.svg`, `.html` |
 | `react`, `react-dom` | Every other framework, plus `react-router-dom` (routing is the host's), `react-native`, `@vitejs/plugin-react` |
 | `@ekklesia/module-ui-sdk`, `@ekklesia/module-ui-build`, `@ekklesia/design-tokens`, `@ekklesia/ui` | Every other shared role: `api-client`, `auth-client`, `surface-sdk`, `module-ui-host` |
 
@@ -581,8 +666,9 @@ Four things about it matter to you as a module author:
 - **Only the artifact that role declares is served.** The module id and version in either URL are
   lookup keys, never path components — there is no way to request a second file from your package.
   Your source map, any sibling chunk, and every package-root file stay unreachable. If your UI needs
-  an asset, it must be inside the bundle; there is no sidecar contract yet, which is also why a
-  stylesheet import still fails the build.
+  an asset, it must be inside the bundle: that is exactly why your stylesheet is compiled into the
+  artifact as text rather than published beside it (ADR 0040), and why a bundled image or font is
+  still not supported.
 - **The version must match exactly.** A request for any other version is refused rather than served
   your installed artifact, so a stale host cannot mount something it did not ask for.
 - **Your artifact bytes are public.** These two routes require no authentication — fetching code
diff --git a/packages/module-ui-build/src/build-module-ui.ts b/packages/module-ui-build/src/build-module-ui.ts
index e9e704f..7ddfc29 100644
--- a/packages/module-ui-build/src/build-module-ui.ts
+++ b/packages/module-ui-build/src/build-module-ui.ts
@@ -23,6 +23,8 @@ export type ModuleUiBuildErrorCode =
   | "MODULE_UI_ENTRYPOINT_UNSAFE"
   | "MODULE_UI_ENTRYPOINT_COLLISION"
   | "MODULE_UI_SOURCE_MISSING"
+  | "MODULE_UI_STYLESHEET_INVALID"
+  | "MODULE_UI_STYLESHEET_UNDECLARED"
   | "MODULE_UI_BUILD_FAILED";
 
 export type ModuleUiArtifactKind = "ui" | "display";
@@ -34,6 +36,10 @@ const errorMessages: Readonly<Record<ModuleUiBuildErrorCode, string>> = Object.f
   MODULE_UI_ENTRYPOINT_COLLISION:
     "A display UI entrypoint must not overwrite another declared artifact.",
   MODULE_UI_SOURCE_MISSING: "A declared module UI source entrypoint was not found.",
+  MODULE_UI_STYLESHEET_INVALID:
+    "A module stylesheet may not define a global Ekklesia theme variable or keep an unresolved @import.",
+  MODULE_UI_STYLESHEET_UNDECLARED:
+    "A module UI build produced a stylesheet beside its artifact. Every style must be compiled into the declared artifact.",
   MODULE_UI_BUILD_FAILED: "A module UI bundle could not be produced.",
 });
 
@@ -58,32 +64,97 @@ export class ModuleUiBuildError extends Error {
 }
 
 /**
- * Rejects a stylesheet import instead of quietly emitting one.
- *
- * esbuild would happily bundle `import "./styles.css"` into a sidecar `index.css` next to the
- * artifact. Nothing declares that file, no host loads it, and the manifest has no field for it — so
- * it would work in a local test and silently do nothing in a real host. Module styling has no
- * public loading contract yet (ADR 0023), so the build fails loudly rather than producing an
- * undeclared asset. `scripts/check-boundaries.ts` rejects a `.css` file authored *inside* a module;
- * this additionally covers a stylesheet reached through a dependency.
+ * A CSS custom property in Ekklesia's own global theme namespace, being **declared** rather than
+ * referenced. `var(--ekklesia-x)` and `var(--ekklesia-x, fallback)` are reads and are fine; only a
+ * `--ekklesia-x:` declaration matches, because that is what would invent a global theme variable
+ * outside the registry a future theme editor enumerates (ADR 0039).
  */
-function rejectStylesheetImports(): Plugin {
+const GLOBAL_THEME_DECLARATION = /--ekklesia-[a-z0-9-]*\s*:/iu;
+
+/** An `@import` that survived compilation — a remote or otherwise unresolvable one. A local import
+ * is inlined by the nested bundle below and leaves nothing behind, so anything still here would make
+ * the artifact fetch a stylesheet at run time, which is exactly what "self-contained" excludes. */
+const SURVIVING_IMPORT = /@import\s/iu;
+
+/**
+ * Compiles each imported stylesheet into the artifact instead of emitting a sidecar.
+ *
+ * esbuild's default for `import "./styles.css"` from JavaScript is to collect every such stylesheet
+ * into an `index.css` output beside the artifact. Nothing declares that file, no host fetches it,
+ * and the manifest has no field for it — it would work in a local test and silently do nothing in a
+ * real host. ADR 0023 therefore rejected the import outright; ADR 0040 compiles it instead.
+ *
+ * Each `.css` import becomes a JavaScript module whose default export is that stylesheet's compiled
+ * text, produced by a nested esbuild run so local `@import`s are resolved and the result is one
+ * self-contained string. The author passes that string to `defineModuleUi({ styles })` and the host
+ * installs it into the artifact's isolated root. The declared `.js` artifact therefore remains the
+ * only file the platform publishes, exactly as before stylesheets existed.
+ *
+ * Other stylesheet dialects stay rejected. Adding one is a language and toolchain decision, not a
+ * detail of this pass, and would need its own ADR.
+ */
+const STYLESHEET_PLUGIN_NAME = "ekklesia-compile-stylesheet-imports";
+
+function compileStylesheetImports(): Plugin {
+  const decoder = new TextDecoder();
   return {
-    name: "ekklesia-reject-stylesheet-imports",
+    name: STYLESHEET_PLUGIN_NAME,
     setup(pluginBuild) {
       // No `u` flag, deliberately: esbuild compiles a plugin filter to a Go RE2 expression, which
-      // rejects `(?iu)` outright ("filter is not a valid Go regular expression"). This pattern is
+      // rejects `(?iu)` outright ("filter is not a valid Go regular expression"). These patterns are
       // pure ASCII, so the flag would change nothing anyway.
-      pluginBuild.onResolve({ filter: /\.(css|s[ac]ss|less|styl|pcss|postcss)$/i }, (args) => ({
+      pluginBuild.onResolve({ filter: /\.(s[ac]ss|less|styl|pcss|postcss)$/i }, (args) => ({
         errors: [
           {
             text:
-              "Module UI cannot import a stylesheet yet: there is no declared way for a host to " +
-              "load one. Use React inline style objects for now (ADR 0023).",
+              "Module UI supports plain .css only. Other stylesheet dialects have no compilation " +
+              "contract in this platform (ADR 0040).",
             location: { file: args.importer },
           },
         ],
       }));
+
+      pluginBuild.onLoad({ filter: /\.css$/i }, async (args) => {
+        const compiled = await build({
+          bundle: true,
+          entryPoints: [args.path],
+          logLevel: "silent",
+          minify: false,
+          target: MODULE_UI_BUILD_TARGET,
+          write: false,
+        });
+        const stylesheet = compiled.outputFiles
+          .map((outputFile) => decoder.decode(outputFile.contents))
+          .join("\n");
+
+        if (GLOBAL_THEME_DECLARATION.test(stylesheet)) {
+          return {
+            errors: [
+              {
+                text:
+                  "A module stylesheet may reference Ekklesia's global theme variables but must " +
+                  "never declare one. Derive a private alias under your own prefix instead " +
+                  "(ADR 0039).",
+                location: { file: args.path },
+              },
+            ],
+          };
+        }
+        if (SURVIVING_IMPORT.test(stylesheet)) {
+          return {
+            errors: [
+              {
+                text:
+                  "A module stylesheet left an unresolved @import. A declared artifact must be " +
+                  "self-contained, so every import has to resolve to a local file (ADR 0040).",
+                location: { file: args.path },
+              },
+            ],
+          };
+        }
+
+        return { contents: `export default ${JSON.stringify(stylesheet)};`, loader: "js" };
+      });
     },
   };
 }
@@ -237,7 +308,7 @@ export async function buildModuleUi(options: BuildModuleUiOptions): Promise<Modu
         minify: false,
         outfile: artifact.outputFile,
         platform: "browser",
-        plugins: [rejectStylesheetImports()],
+        plugins: [compileStylesheetImports()],
         sourcemap: true,
         target: MODULE_UI_BUILD_TARGET,
         // Fail rather than silently shipping a partial bundle.
@@ -246,6 +317,17 @@ export async function buildModuleUi(options: BuildModuleUiOptions): Promise<Modu
         // atomic with respect to parse/resolve/bundle failures rather than leaving a stale partial set.
         write: false,
       });
+      // Belt and braces on the stylesheet contract. The plugin above turns every `.css` import into
+      // JavaScript, so esbuild has nothing left to emit a stylesheet from — but a future option or
+      // loader change could reintroduce a sidecar, and a sidecar is precisely the failure this
+      // design exists to make impossible: a file nothing declares, no host fetches, and every local
+      // test silently passes without. Producing one fails the build rather than shipping it.
+      const sidecar = result.outputFiles.find(
+        (outputFile) => path.extname(outputFile.path).toLowerCase() === ".css",
+      );
+      if (sidecar) {
+        throw new ModuleUiBuildError("MODULE_UI_STYLESHEET_UNDECLARED", artifact.kind);
+      }
       pendingWrites.push(
         result.outputFiles.map((outputFile) => ({
           contents: outputFile.contents,
@@ -253,10 +335,21 @@ export async function buildModuleUi(options: BuildModuleUiOptions): Promise<Modu
         })),
       );
     } catch (error) {
+      if (error instanceof ModuleUiBuildError) {
+        throw error;
+      }
       // esbuild's structured diagnostics are surfaced by the CLI; rethrow a sanitized error so a
       // programmatic caller never receives raw paths or internals.
       if (isRecord(error) && Array.isArray(error.errors)) {
-        throw Object.assign(new ModuleUiBuildError("MODULE_UI_BUILD_FAILED", artifact.kind), {
+        // A stylesheet the module authored broke one of this platform's own rules, rather than
+        // failing to compile. Reported with its own code so a caller can tell "your CSS is not
+        // allowed to do that" from "your TSX does not compile".
+        const code = error.errors.some(
+          (diagnostic) => isRecord(diagnostic) && diagnostic.pluginName === STYLESHEET_PLUGIN_NAME,
+        )
+          ? "MODULE_UI_STYLESHEET_INVALID"
+          : "MODULE_UI_BUILD_FAILED";
+        throw Object.assign(new ModuleUiBuildError(code, artifact.kind), {
           diagnostics: error.errors,
         });
       }
diff --git a/packages/module-ui-build/tests/build-module-ui.test.ts b/packages/module-ui-build/tests/build-module-ui.test.ts
index 346c170..57f623c 100644
--- a/packages/module-ui-build/tests/build-module-ui.test.ts
+++ b/packages/module-ui-build/tests/build-module-ui.test.ts
@@ -342,10 +342,10 @@ describe("module UI build", () => {
   });
 
   it.each([
-    "styles.css",
     "theme.scss",
     "layout.less",
-  ])("rejects a %s import rather than emitting an undeclared sidecar asset", async (stylesheet) => {
+    "tokens.styl",
+  ])("rejects a %s import: only plain CSS has a compilation contract", async (stylesheet) => {
     const directory = await createModule({
       ui: "./dist/ui/index.js",
       files: {
@@ -358,47 +358,131 @@ describe("module UI build", () => {
       await buildModuleUi({ moduleDirectory: directory });
       expect.unreachable("expected the stylesheet import to be rejected");
     } catch (error) {
-      expect((error as ModuleUiBuildError).code).toBe("MODULE_UI_BUILD_FAILED");
+      expect((error as ModuleUiBuildError).code).toBe("MODULE_UI_STYLESHEET_INVALID");
       const diagnostics = (error as { diagnostics?: { text?: string }[] }).diagnostics ?? [];
-      expect(diagnostics.some((entry) => entry.text?.includes("cannot import a stylesheet"))).toBe(
-        true,
-      );
+      expect(diagnostics.some((entry) => entry.text?.includes("plain .css only"))).toBe(true);
     }
   });
 
-  it("never leaves a stylesheet sidecar next to the artifact", async () => {
+  it("compiles a plain stylesheet into the artifact instead of emitting a sidecar", async () => {
     const directory = await createModule({
       ui: "./dist/ui/index.js",
       files: {
-        "src/ui/index.tsx": 'import "./styles.css";\nexport default {};',
-        "src/ui/styles.css": ".example { color: red; }",
+        "src/ui/index.tsx": `
+          import styles from "./styles.css";
+          export default { id: "example", version: "0.1.0", styles, mount: () => undefined };`,
+        "src/ui/styles.css": ".example { color: var(--ekklesia-text-primary); }",
       },
     });
 
-    await expect(buildModuleUi({ moduleDirectory: directory })).rejects.toBeInstanceOf(
-      ModuleUiBuildError,
-    );
+    const result = await buildModuleUi({ moduleDirectory: directory });
+    const artifact = await readFile(result.artifacts[0]?.outputFile ?? "", "utf8");
+
+    // The whole stylesheet travels inside the one declared artifact.
+    expect(artifact).toContain(".example");
+    expect(artifact).toContain("--ekklesia-text-primary");
+    // And nothing was emitted beside it. A sidecar is the exact failure this design prevents: a
+    // file nothing declares, no host fetches, and every local test silently passes without.
     await expect(
       readFile(path.join(directory, "dist", "ui", "index.css"), "utf8"),
     ).rejects.toThrow();
   });
 
-  it("enforces the stylesheet boundary for display UI too", async () => {
+  it("resolves a local @import so the compiled stylesheet is self-contained", async () => {
+    const directory = await createModule({
+      ui: "./dist/ui/index.js",
+      files: {
+        "src/ui/index.tsx": `
+          import styles from "./styles.css";
+          export default { id: "example", version: "0.1.0", styles, mount: () => undefined };`,
+        "src/ui/styles.css": '@import "./base.css";\n.example { color: red; }',
+        "src/ui/base.css": ".base { display: grid; }",
+      },
+    });
+
+    const result = await buildModuleUi({ moduleDirectory: directory });
+    const artifact = await readFile(result.artifacts[0]?.outputFile ?? "", "utf8");
+
+    expect(artifact).toContain(".base");
+    expect(artifact).toContain(".example");
+  });
+
+  it("rejects a stylesheet that keeps an unresolved @import, so the artifact stays self-contained", async () => {
+    const directory = await createModule({
+      ui: "./dist/ui/index.js",
+      files: {
+        "src/ui/index.tsx": `
+          import styles from "./styles.css";
+          export default { id: "example", version: "0.1.0", styles, mount: () => undefined };`,
+        "src/ui/styles.css": '@import url("https://fonts.example.invalid/family.css");',
+      },
+    });
+
+    await expect(buildModuleUi({ moduleDirectory: directory })).rejects.toMatchObject({
+      code: "MODULE_UI_STYLESHEET_INVALID",
+    });
+  });
+
+  it("rejects a stylesheet that declares a global Ekklesia theme variable", async () => {
+    const directory = await createModule({
+      ui: "./dist/ui/index.js",
+      files: {
+        "src/ui/index.tsx": `
+          import styles from "./styles.css";
+          export default { id: "example", version: "0.1.0", styles, mount: () => undefined };`,
+        // Referencing a token is the contract; *declaring* one would invent a global theme variable
+        // outside the registry a future theme editor enumerates (ADR 0039).
+        "src/ui/styles.css": ":host { --ekklesia-text-primary: hotpink; }",
+      },
+    });
+
+    try {
+      await buildModuleUi({ moduleDirectory: directory });
+      expect.unreachable("expected the stylesheet to be rejected");
+    } catch (error) {
+      expect((error as ModuleUiBuildError).code).toBe("MODULE_UI_STYLESHEET_INVALID");
+      const diagnostics = (error as { diagnostics?: { text?: string }[] }).diagnostics ?? [];
+      expect(diagnostics.some((entry) => entry.text?.includes("never declare one"))).toBe(true);
+    }
+  });
+
+  it("accepts a private alias derived from a registered token", async () => {
+    const directory = await createModule({
+      ui: "./dist/ui/index.js",
+      files: {
+        "src/ui/index.tsx": `
+          import styles from "./styles.css";
+          export default { id: "example", version: "0.1.0", styles, mount: () => undefined };`,
+        "src/ui/styles.css":
+          ":host { --example-gap: var(--ekklesia-spacing-sm); }\n.example { gap: var(--example-gap); }",
+      },
+    });
+
+    const artifact = await readFile(
+      (await buildModuleUi({ moduleDirectory: directory })).artifacts[0]?.outputFile ?? "",
+      "utf8",
+    );
+    expect(artifact).toContain("--example-gap");
+  });
+
+  it("compiles a display artifact's stylesheet the same way", async () => {
     const directory = await createModule({
       display: {
         artifact: "./dist/display/index.js",
         permission: "example.display.view",
       },
       files: {
-        "src/display/index.tsx": 'import "./styles.css";\nexport default {};',
+        "src/display/index.tsx": `
+          import styles from "./styles.css";
+          export default { id: "example", version: "0.1.0", styles, mount: () => undefined };`,
         "src/display/styles.css": ".display { position: fixed; }",
       },
     });
 
-    await expect(buildModuleUi({ moduleDirectory: directory })).rejects.toMatchObject({
-      code: "MODULE_UI_BUILD_FAILED",
-      kind: "display",
-    });
+    const result = await buildModuleUi({ moduleDirectory: directory });
+    const artifact = await readFile(result.artifacts[0]?.outputFile ?? "", "utf8");
+
+    expect(artifact).toContain(".display");
     await expect(readFile(path.join(directory, "dist/display/index.css"))).rejects.toThrow();
   });
 
diff --git a/packages/module-ui-host/src/index.ts b/packages/module-ui-host/src/index.ts
index 0cc9f60..fcfc65e 100644
--- a/packages/module-ui-host/src/index.ts
+++ b/packages/module-ui-host/src/index.ts
@@ -1 +1,2 @@
+export * from "./isolated-root.js";
 export * from "./load-module-ui.js";
diff --git a/packages/module-ui-host/src/isolated-root.ts b/packages/module-ui-host/src/isolated-root.ts
new file mode 100644
index 0000000..e1f861a
--- /dev/null
+++ b/packages/module-ui-host/src/isolated-root.ts
@@ -0,0 +1,108 @@
+/**
+ * The isolated root a host mounts a browser artifact into (ADR 0040).
+ *
+ * Every declared browser artifact — a module's primary UI, its display UI, and a contributed
+ * renderer — is mounted the same way, into its own Shadow DOM root the loader creates and owns:
+ *
+ * ```text
+ * host container            ← the host's element; the loader never mutates it beyond this child
+ * └─ <div data-ekklesia-module-ui>            ← shadow host, created and removed by the loader
+ *      #shadow-root (open)
+ *        ├─ <style data-ekklesia-module-ui-styles>  base rules + the artifact's declared CSS
+ *        └─ <div data-ekklesia-module-ui-root>      ← the Element handed to mount()
+ * ```
+ *
+ * Shadow DOM is chosen because it is the one mechanism in a browser that actually **enforces** the
+ * rule rather than describing it: rules inside a shadow root do not apply outside it, and document
+ * rules do not apply inside it. A class-name convention, a build-time prefix, or a "please scope
+ * your selectors" instruction are all conventions a single mistake defeats, and the failure mode is
+ * a module silently restyling the Dashboard or another module's renderer.
+ *
+ * It is **not** a security boundary, and nothing here should be read as one. `mode: "open"` is
+ * deliberate: an installed module is trusted first-party code running in the host's own realm
+ * (ADR 0022, ADR 0024), it could reach `document` directly if it wanted to, and `mode: "closed"`
+ * would buy no protection while making the artifact's DOM unreachable to the host's own tests and
+ * diagnostics. This is a correctness boundary for *styles*, not a sandbox.
+ *
+ * Custom properties inherit through a shadow boundary, so the design tokens a host installed at its
+ * root (ADR 0039) are available inside every artifact with nothing passed and nothing imported.
+ */
+
+export const MODULE_UI_ISOLATION_ATTRIBUTE = "data-ekklesia-module-ui";
+export const MODULE_UI_ISOLATION_STYLE_ATTRIBUTE = "data-ekklesia-module-ui-styles";
+export const MODULE_UI_ISOLATION_ROOT_ATTRIBUTE = "data-ekklesia-module-ui-root";
+
+/**
+ * The rules the host always installs, before any artifact CSS.
+ *
+ * Both wrappers are `display: contents`, so the two elements the loader adds contribute no boxes of
+ * their own and an artifact's own top-level elements lay out inside the host's container exactly as
+ * they did before isolation existed. They are declared as CSS rather than inline styles precisely so
+ * an artifact's own stylesheet — which the host appends *after* these — can override them when it
+ * genuinely needs a box to size.
+ */
+const BASE_STYLESHEET = `:host{display:contents}\n[${MODULE_UI_ISOLATION_ROOT_ATTRIBUTE}]{display:contents}`;
+
+export interface IsolatedArtifactRoot {
+  /** The element handed to the artifact's `mount`. */
+  readonly mountTarget: Element;
+  /** The open shadow root, for a host's own tests and diagnostics. */
+  readonly shadowRoot: ShadowRoot;
+  /** Removes the whole isolated root — content and styles together. Idempotent. */
+  dispose(): void;
+}
+
+/**
+ * Builds one isolated root inside `container` and returns it.
+ *
+ * Nothing is attached to `container` until the shadow root, its stylesheet, and its mount target
+ * have all been created, so a host whose environment cannot provide Shadow DOM gets a thrown error
+ * and an untouched container rather than a half-built, unisolated mount.
+ *
+ * Each call creates a **fresh** root, and `dispose` removes it whole. That is what makes styles
+ * deterministic in both directions: a mount installs exactly one `<style>`, an unmount removes it
+ * with the element that owns it, and remounting cannot accumulate a second copy because there is
+ * nothing left to accumulate onto.
+ */
+export function createIsolatedArtifactRoot(
+  container: Element,
+  styles: string | undefined,
+): IsolatedArtifactRoot {
+  const ownerDocument = container.ownerDocument;
+  if (!ownerDocument) {
+    throw new TypeError("A module UI container must belong to a document.");
+  }
+
+  const shadowHost = ownerDocument.createElement("div");
+  shadowHost.setAttribute(MODULE_UI_ISOLATION_ATTRIBUTE, "");
+  if (typeof shadowHost.attachShadow !== "function") {
+    throw new TypeError("This environment does not support Shadow DOM.");
+  }
+  const shadowRoot = shadowHost.attachShadow({ mode: "open" });
+
+  const style = ownerDocument.createElement("style");
+  style.setAttribute(MODULE_UI_ISOLATION_STYLE_ATTRIBUTE, "");
+  // The artifact's own CSS comes last, so it wins over the base rules above by ordinary cascade
+  // order rather than by anything the host has to special-case.
+  style.textContent = styles === undefined ? BASE_STYLESHEET : `${BASE_STYLESHEET}\n${styles}`;
+  shadowRoot.appendChild(style);
+
+  const mountTarget = ownerDocument.createElement("div");
+  mountTarget.setAttribute(MODULE_UI_ISOLATION_ROOT_ATTRIBUTE, "");
+  shadowRoot.appendChild(mountTarget);
+
+  container.appendChild(shadowHost);
+
+  let disposed = false;
+  return {
+    dispose: () => {
+      if (disposed) {
+        return;
+      }
+      disposed = true;
+      shadowHost.remove();
+    },
+    mountTarget,
+    shadowRoot,
+  };
+}
diff --git a/packages/module-ui-host/src/load-module-ui.ts b/packages/module-ui-host/src/load-module-ui.ts
index 4984333..d2e9b33 100644
--- a/packages/module-ui-host/src/load-module-ui.ts
+++ b/packages/module-ui-host/src/load-module-ui.ts
@@ -4,6 +4,7 @@ import {
   type ModuleUiUnmount,
   parseModuleUiExport,
 } from "@ekklesia/module-ui-sdk";
+import { createIsolatedArtifactRoot, type IsolatedArtifactRoot } from "./isolated-root.js";
 
 /**
  * How a host obtains a module's UI artifact.
@@ -45,7 +46,11 @@ const defaultImporter: ModuleUiImporter = (url) => import(url);
 
 export interface LoadModuleUiOptions {
   readonly source: ModuleUiSource;
-  /** The element the module renders into. The host owns it; the module owns its contents. */
+  /**
+   * The element the loader builds this artifact's isolated root inside. The host owns it, the
+   * module owns its own root's contents, and the loader adds and removes exactly one child of its
+   * own (ADR 0040). The artifact never receives this element.
+   */
   readonly container: Element;
   /** Supplied whole by the host. This package forwards it and never adds to it. */
   readonly context: ModuleUiHostContext;
@@ -61,16 +66,28 @@ export interface MountedModuleUi {
   /** Whether cleanup is still pending. Becomes `false` after the first {@link unmount}. */
   readonly mounted: boolean;
   /**
-   * Runs the module's cleanup, if it returned one. Idempotent: the first call cleans up, later
+   * This artifact's open shadow root. Exposed so a host's own tests and diagnostics can see what an
+   * artifact actually rendered; a host has no reason to write to it, and doing so would be reaching
+   * into content the module owns.
+   */
+  readonly shadowRoot: ShadowRoot;
+  /**
+   * Runs the module's cleanup, if it returned one, then removes the isolated root the loader
+   * created — content and installed styles together. Idempotent: the first call cleans up, later
    * calls do nothing. Safe when the module returned no cleanup function.
    */
   unmount(): Promise<void>;
 }
 
-export type ModuleUiHostErrorCode = "MODULE_UI_IMPORT_FAILED" | "MODULE_UI_MOUNT_FAILED";
+export type ModuleUiHostErrorCode =
+  | "MODULE_UI_IMPORT_FAILED"
+  | "MODULE_UI_ISOLATION_UNAVAILABLE"
+  | "MODULE_UI_MOUNT_FAILED";
 
 const errorMessages: Readonly<Record<ModuleUiHostErrorCode, string>> = Object.freeze({
   MODULE_UI_IMPORT_FAILED: "A module UI artifact could not be imported from its published URL.",
+  MODULE_UI_ISOLATION_UNAVAILABLE:
+    "A module UI artifact could not be given an isolated root in this environment.",
   MODULE_UI_MOUNT_FAILED: "A module UI artifact failed while mounting.",
 });
 
@@ -103,6 +120,12 @@ export class ModuleUiHostError extends Error {
  *
  * Nothing is mounted until the artifact's declared identity matches the manifest identity the host
  * passed in, so a stale or mismatched bundle fails before it can render or run `mount`.
+ *
+ * Every artifact is mounted into its own isolated root the loader creates inside `container`
+ * (ADR 0040), and the artifact's declared `styles` are installed there. That is what keeps one
+ * module's stylesheet out of the Dashboard, out of another module, and out of another contributed
+ * renderer — and it applies identically to every role, so there is one mounting path rather than a
+ * styled one and an unstyled one.
  */
 export async function loadModuleUi(options: LoadModuleUiOptions): Promise<MountedModuleUi> {
   const { container, context, source } = options;
@@ -124,16 +147,26 @@ export async function loadModuleUi(options: LoadModuleUiOptions): Promise<Mounte
     version: source.version,
   });
 
+  // Built before `mount` runs and torn down again if `mount` throws, so a failed artifact leaves
+  // neither DOM nor styles behind.
+  let isolated: IsolatedArtifactRoot;
+  try {
+    isolated = createIsolatedArtifactRoot(container, definition.styles);
+  } catch (error) {
+    throw new ModuleUiHostError("MODULE_UI_ISOLATION_UNAVAILABLE", { cause: error });
+  }
+
   let cleanup: ModuleUiUnmount | undefined;
   try {
-    const result = await definition.mount(container, context);
+    const result = await definition.mount(isolated.mountTarget, context);
     if (typeof result === "function") {
       cleanup = result;
     }
   } catch (error) {
     // No cleanup is registered for a mount that never completed, so the returned handle can never
-    // claim a failed module is mounted. The container is deliberately left untouched: the host owns
-    // it and decides what a failed mount should look like.
+    // claim a failed module is mounted. The host's own container is deliberately left as it was:
+    // the host owns it and decides what a failed mount should look like.
+    isolated.dispose();
     throw new ModuleUiHostError("MODULE_UI_MOUNT_FAILED", { cause: error });
   }
 
@@ -144,14 +177,20 @@ export async function loadModuleUi(options: LoadModuleUiOptions): Promise<Mounte
     get mounted() {
       return mounted;
     },
+    shadowRoot: isolated.shadowRoot,
     async unmount() {
       if (!mounted) {
         return;
       }
       // Marked unmounted before running cleanup, so a throwing cleanup surfaces to the host once
-      // and a second call never re-runs it.
+      // and a second call never re-runs it. The isolated root is removed in a `finally`, so a
+      // module whose cleanup throws still has its styles taken out of the page.
       mounted = false;
-      await cleanup?.();
+      try {
+        await cleanup?.();
+      } finally {
+        isolated.dispose();
+      }
     },
   };
 }
diff --git a/packages/module-ui-host/tests/fixtures/artifact-module/module-styles.d.ts b/packages/module-ui-host/tests/fixtures/artifact-module/module-styles.d.ts
new file mode 100644
index 0000000..ecabc74
--- /dev/null
+++ b/packages/module-ui-host/tests/fixtures/artifact-module/module-styles.d.ts
@@ -0,0 +1,4 @@
+declare module "*.css" {
+  const stylesheet: string;
+  export default stylesheet;
+}
diff --git a/packages/module-ui-host/tests/fixtures/artifact-module/src/ui/index.tsx b/packages/module-ui-host/tests/fixtures/artifact-module/src/ui/index.tsx
index 120dfed..abfa5d8 100644
--- a/packages/module-ui-host/tests/fixtures/artifact-module/src/ui/index.tsx
+++ b/packages/module-ui-host/tests/fixtures/artifact-module/src/ui/index.tsx
@@ -1,15 +1,19 @@
 import { defineModuleUi } from "@ekklesia/module-ui-sdk";
+import styles from "./styles.css";
 
 /**
  * A real module UI compiled by the canonical pipeline. It imports the public SDK so the built
  * artifact proves the whole loop closes: SDK source -> esbuild bundle -> browser URL -> host loader.
  *
  * React is deliberately absent — the module template already proves the React path end to end, and
- * omitting it keeps this artifact small enough to serve as a `data:` URL.
+ * omitting it keeps this artifact small enough to serve as a `data:` URL. It does import a local
+ * stylesheet, so the built artifact also proves the CSS half of that loop: `.css` source -> compiled
+ * text inside the declared artifact -> installed in the isolated root at mount (ADR 0040).
  */
 export default defineModuleUi({
   id: "example",
   version: "0.1.0",
+  styles,
   async mount(container, context) {
     const response = await context.api.request("records");
     const label = context.principal.kind === "authenticated" ? context.principal.userId : "guest";
diff --git a/packages/module-ui-host/tests/fixtures/artifact-module/src/ui/styles.css b/packages/module-ui-host/tests/fixtures/artifact-module/src/ui/styles.css
new file mode 100644
index 0000000..1cae7f9
--- /dev/null
+++ b/packages/module-ui-host/tests/fixtures/artifact-module/src/ui/styles.css
@@ -0,0 +1,5 @@
+/* Compiled into the artifact by the canonical builder and installed into its isolated root by the
+ * loader (ADR 0040). It references a global token and declares none of its own. */
+.artifact-label {
+  color: var(--ekklesia-text-primary);
+}
diff --git a/packages/module-ui-host/tests/load-module-ui.test.ts b/packages/module-ui-host/tests/load-module-ui.test.ts
index 06c657d..46b80a2 100644
--- a/packages/module-ui-host/tests/load-module-ui.test.ts
+++ b/packages/module-ui-host/tests/load-module-ui.test.ts
@@ -1,7 +1,18 @@
 // @vitest-environment happy-dom
-import { ModuleUiContractError, type ModuleUiHostContext } from "@ekklesia/module-ui-sdk";
+import {
+  MODULE_UI_ABI_VERSION,
+  ModuleUiContractError,
+  type ModuleUiHostContext,
+} from "@ekklesia/module-ui-sdk";
 import { describe, expect, it } from "vitest";
-import { loadModuleUi, ModuleUiHostError, type ModuleUiImporter } from "../src/index.js";
+import {
+  loadModuleUi,
+  MODULE_UI_ISOLATION_ATTRIBUTE,
+  MODULE_UI_ISOLATION_ROOT_ATTRIBUTE,
+  MODULE_UI_ISOLATION_STYLE_ATTRIBUTE,
+  ModuleUiHostError,
+  type ModuleUiImporter,
+} from "../src/index.js";
 
 /**
  * A browser-loadable URL for a module built from source text.
@@ -15,10 +26,14 @@ function moduleUrl(source: string): string {
   return `data:text/javascript;base64,${Buffer.from(source, "utf8").toString("base64")}`;
 }
 
-const validArtifact = (options: { id?: string; version?: string; cleanup?: boolean } = {}) => `
+const validArtifact = (
+  options: { id?: string; version?: string; cleanup?: boolean; styles?: string } = {},
+) => `
 export default {
+  abi: ${MODULE_UI_ABI_VERSION},
   id: ${JSON.stringify(options.id ?? "announcements")},
   version: ${JSON.stringify(options.version ?? "1.2.0")},
+  ${options.styles === undefined ? "" : `styles: ${JSON.stringify(options.styles)},`}
   mount(container, context) {
     globalThis.__mounts ??= [];
     globalThis.__mounts.push({ moduleId: context.moduleId, container });
@@ -54,6 +69,20 @@ function container(): Element {
   return element;
 }
 
+/**
+ * What the artifact actually rendered, reached the way a host's own diagnostics would: through the
+ * open shadow root the loader created. The host's container itself holds only the loader's one
+ * wrapper element, which is exactly the isolation ADR 0040 introduced.
+ */
+function artifactRoot(element: Element): Element | null {
+  const shadowHost = element.querySelector(`[${MODULE_UI_ISOLATION_ATTRIBUTE}]`);
+  return shadowHost?.shadowRoot?.querySelector(`[${MODULE_UI_ISOLATION_ROOT_ATTRIBUTE}]`) ?? null;
+}
+
+function artifactText(element: Element): string {
+  return artifactRoot(element)?.textContent ?? "";
+}
+
 const source = { moduleId: "announcements", version: "1.2.0" };
 
 describe("generic module UI host loader", () => {
@@ -70,10 +99,99 @@ describe("generic module UI host loader", () => {
     expect(mounted.moduleId).toBe("announcements");
     expect(mounted.version).toBe("1.2.0");
     expect(mounted.mounted).toBe(true);
-    expect(element.textContent).toBe("mounted:announcements");
+    expect(artifactText(element)).toBe("mounted:announcements");
   });
 
-  it("passes the host's own container and context straight through", async () => {
+  it("mounts the artifact into an isolated root rather than the host's own container", async () => {
+    const element = container();
+
+    const mounted = await loadModuleUi({
+      source: { ...source, url: moduleUrl(validArtifact()) },
+      container: element,
+      context: hostContext([]),
+    });
+
+    // The host's container gained exactly one child: the loader's shadow host.
+    expect(element.children).toHaveLength(1);
+    const shadowHost = element.firstElementChild as Element;
+    expect(shadowHost.hasAttribute(MODULE_UI_ISOLATION_ATTRIBUTE)).toBe(true);
+    expect(shadowHost.shadowRoot).toBe(mounted.shadowRoot);
+    // Open, deliberately: this is a correctness boundary for styles, not a sandbox — an installed
+    // module is trusted code in the host's own realm and could reach `document` regardless.
+    expect(mounted.shadowRoot.mode).toBe("open");
+    // The artifact's DOM is not reachable from the host's own document tree.
+    expect(element.textContent).toBe("");
+  });
+
+  it("installs a declared stylesheet inside the isolated root, exactly once", async () => {
+    const element = container();
+
+    const mounted = await loadModuleUi({
+      source: { ...source, url: moduleUrl(validArtifact({ styles: ".posts{color:red}" })) },
+      container: element,
+      context: hostContext([]),
+    });
+
+    const styles = mounted.shadowRoot.querySelectorAll(
+      `style[${MODULE_UI_ISOLATION_STYLE_ATTRIBUTE}]`,
+    );
+    expect(styles).toHaveLength(1);
+    expect(styles[0]?.textContent).toContain(".posts{color:red}");
+    // The host document never receives the artifact's rules.
+    expect(document.head.querySelector(`style[${MODULE_UI_ISOLATION_STYLE_ATTRIBUTE}]`)).toBeNull();
+  });
+
+  it("installs the host's base rules before the artifact's own, so an artifact can override them", async () => {
+    const element = container();
+
+    const mounted = await loadModuleUi({
+      source: { ...source, url: moduleUrl(validArtifact({ styles: ":host{display:block}" })) },
+      container: element,
+      context: hostContext([]),
+    });
+
+    const stylesheet =
+      mounted.shadowRoot.querySelector(`style[${MODULE_UI_ISOLATION_STYLE_ATTRIBUTE}]`)
+        ?.textContent ?? "";
+    expect(stylesheet.indexOf(":host{display:contents}")).toBeLessThan(
+      stylesheet.indexOf(":host{display:block}"),
+    );
+  });
+
+  it("still mounts an artifact that declares no stylesheet", async () => {
+    const element = container();
+
+    const mounted = await loadModuleUi({
+      source: { ...source, url: moduleUrl(validArtifact()) },
+      container: element,
+      context: hostContext([]),
+    });
+
+    expect(artifactText(element)).toBe("mounted:announcements");
+    expect(
+      mounted.shadowRoot.querySelectorAll(`style[${MODULE_UI_ISOLATION_STYLE_ATTRIBUTE}]`),
+    ).toHaveLength(1);
+  });
+
+  it("never accumulates styles across repeated mounts of the same container", async () => {
+    const element = container();
+    const url = moduleUrl(validArtifact({ styles: ".posts{color:red}" }));
+
+    for (let attempt = 0; attempt < 3; attempt += 1) {
+      const mounted = await loadModuleUi({
+        source: { ...source, url },
+        container: element,
+        context: hostContext([]),
+      });
+      expect(
+        mounted.shadowRoot.querySelectorAll(`style[${MODULE_UI_ISOLATION_STYLE_ATTRIBUTE}]`),
+      ).toHaveLength(1);
+      await mounted.unmount();
+      expect(element.children).toHaveLength(0);
+    }
+  });
+
+  it("passes the isolated mount target and the host's context straight through", async () => {
     const element = container();
     const requested: string[] = [];
     const context = hostContext(requested);
@@ -86,7 +204,9 @@ describe("generic module UI host loader", () => {
 
     const mounts = (globalThis as { __mounts?: { moduleId: string; container: Element }[] })
       .__mounts;
-    expect(mounts?.at(-1)?.container).toBe(element);
+    // The artifact receives the loader's isolated root, never the host's own element.
+    expect(mounts?.at(-1)?.container).toBe(artifactRoot(element));
+    expect(mounts?.at(-1)?.container).not.toBe(element);
     expect(mounts?.at(-1)?.moduleId).toBe("announcements");
   });
 
@@ -104,7 +224,7 @@ describe("generic module UI host loader", () => {
     expect(requested.join(" ")).not.toContain("/api/v1/modules");
   });
 
-  it("runs the module's cleanup on unmount without touching the host container", async () => {
+  it("runs the module's cleanup and removes the isolated root without touching the host container", async () => {
     const element = container();
     const parent = element.parentElement;
 
@@ -116,7 +236,8 @@ describe("generic module UI host loader", () => {
     await mounted.unmount();
 
     expect(mounted.mounted).toBe(false);
-    expect(element.textContent).toBe("");
+    // The artifact's root, and with it its styles, are gone.
+    expect(element.children).toHaveLength(0);
     // The host owns the container; unmounting a module never removes it.
     expect(element.parentElement).toBe(parent);
     expect(document.body.contains(element)).toBe(true);
@@ -151,13 +272,15 @@ describe("generic module UI host loader", () => {
 
     await expect(mounted.unmount()).resolves.toBeUndefined();
     expect(mounted.mounted).toBe(false);
+    expect(element.children).toHaveLength(0);
   });
 
   it("marks an instance unmounted even when its cleanup throws, so it never re-runs", async () => {
     const element = container();
     const artifact = `
       export default {
-        id: "announcements", version: "1.2.0",
+        abi: ${MODULE_UI_ABI_VERSION}, id: "announcements", version: "1.2.0",
+        styles: ".x{color:red}",
         mount() { return () => { globalThis.__cleanupCalls = (globalThis.__cleanupCalls ?? 0) + 1; throw new Error("cleanup exploded"); }; },
       };`;
 
@@ -169,6 +292,8 @@ describe("generic module UI host loader", () => {
 
     await expect(mounted.unmount()).rejects.toThrow("cleanup exploded");
     expect(mounted.mounted).toBe(false);
+    // A module that fails to clean up must not leave its stylesheet installed in the page.
+    expect(element.children).toHaveLength(0);
     await expect(mounted.unmount()).resolves.toBeUndefined();
     expect((globalThis as { __cleanupCalls?: number }).__cleanupCalls).toBe(1);
   });
@@ -189,16 +314,17 @@ describe("generic module UI host loader", () => {
       context: hostContext([]),
     });
 
-    expect(first.textContent).toBe("mounted:announcements");
-    expect(second.textContent).toBe("mounted:announcements");
+    expect(artifactText(first)).toBe("mounted:announcements");
+    expect(artifactText(second)).toBe("mounted:announcements");
+    expect(a.shadowRoot).not.toBe(b.shadowRoot);
 
     await a.unmount();
 
     // Each handle owns its own cleanup state; unmounting one leaves the other alone.
     expect(a.mounted).toBe(false);
     expect(b.mounted).toBe(true);
-    expect(first.textContent).toBe("");
-    expect(second.textContent).toBe("mounted:announcements");
+    expect(first.children).toHaveLength(0);
+    expect(artifactText(second)).toBe("mounted:announcements");
   });
 
   describe("rejects an artifact before mounting it", () => {
@@ -209,7 +335,7 @@ describe("generic module UI host loader", () => {
       await expect(
         loadModuleUi({ source: { ...source, url }, container: element, context: hostContext([]) }),
       ).rejects.toMatchObject({ code: "MODULE_UI_IDENTITY_MISMATCH" });
-      expect(element.textContent).toBe("");
+      expect(element.children).toHaveLength(0);
     });
 
     it("when the version does not match the manifest", async () => {
@@ -219,7 +345,19 @@ describe("generic module UI host loader", () => {
       await expect(
         loadModuleUi({ source: { ...source, url }, container: element, context: hostContext([]) }),
       ).rejects.toMatchObject({ code: "MODULE_UI_IDENTITY_MISMATCH" });
-      expect(element.textContent).toBe("");
+      expect(element.children).toHaveLength(0);
+    });
+
+    it("when it was built against the previous mount ABI", async () => {
+      const element = container();
+      const url = moduleUrl(
+        'export default { id: "announcements", version: "1.2.0", mount() {} };',
+      );
+
+      await expect(
+        loadModuleUi({ source: { ...source, url }, container: element, context: hostContext([]) }),
+      ).rejects.toMatchObject({ code: "MODULE_UI_ABI_UNSUPPORTED" });
+      expect(element.children).toHaveLength(0);
     });
 
     it("when there is no default export", async () => {
@@ -233,7 +371,9 @@ describe("generic module UI host loader", () => {
 
     it("when the default export is not a module UI definition", async () => {
       const element = container();
-      const url = moduleUrl('export default { id: "announcements", version: "1.2.0" };');
+      const url = moduleUrl(
+        `export default { abi: ${MODULE_UI_ABI_VERSION}, id: "announcements", version: "1.2.0" };`,
+      );
 
       const failure = loadModuleUi({
         source: { ...source, url },
@@ -263,11 +403,12 @@ describe("generic module UI host loader", () => {
     }
   });
 
-  it("reports a throwing mount as a mount failure and registers no cleanup", async () => {
+  it("reports a throwing mount as a mount failure, registers no cleanup, and removes the root", async () => {
     const element = container();
     const url = moduleUrl(`
       export default {
-        id: "announcements", version: "1.2.0",
+        abi: ${MODULE_UI_ABI_VERSION}, id: "announcements", version: "1.2.0",
+        styles: ".x{color:red}",
         mount(container) { container.textContent = "partial"; throw new Error("mount exploded"); },
       };`);
 
@@ -284,9 +425,38 @@ describe("generic module UI host loader", () => {
       expect((error as ModuleUiHostError).message).not.toContain("exploded");
       expect((error as { cause?: Error }).cause?.message).toBe("mount exploded");
     }
-    // No handle exists, so nothing can claim the module is mounted. The container is left exactly
-    // as the module left it: the host owns it and decides what a failed mount looks like.
-    expect(element.textContent).toBe("partial");
+    // No handle exists, so nothing can claim the module is mounted — and a failed mount leaves
+    // neither partial DOM nor an installed stylesheet behind in the host's container.
+    expect(element.children).toHaveLength(0);
+  });
+
+  it("reports an environment with no Shadow DOM as an isolation failure, never as an unisolated mount", async () => {
+    const element = container();
+    const detached = {
+      ownerDocument: document,
+      appendChild: element.appendChild.bind(element),
+    } as unknown as Element;
+    // A container whose created element cannot take a shadow root: the loader must refuse rather
+    // than fall back to mounting the artifact — and its styles — straight into the host's page.
+    const noShadow = new Proxy(detached, {
+      get(target, property, receiver) {
+        if (property === "ownerDocument") {
+          return {
+            createElement: () => ({ setAttribute: () => undefined }),
+          };
+        }
+        return Reflect.get(target, property, receiver);
+      },
+    });
+
+    await expect(
+      loadModuleUi({
+        source: { ...source, url: moduleUrl(validArtifact()) },
+        container: noShadow,
+        context: hostContext([]),
+      }),
+    ).rejects.toMatchObject({ code: "MODULE_UI_ISOLATION_UNAVAILABLE" });
+    expect(element.children).toHaveLength(0);
   });
 
   it("accepts a host-supplied importer for bundlers that need their own escape hatch", async () => {
@@ -294,7 +464,14 @@ describe("generic module UI host loader", () => {
     const seen: string[] = [];
     const importModule: ModuleUiImporter = async (url) => {
       seen.push(url);
-      return { default: { id: "announcements", version: "1.2.0", mount: () => undefined } };
+      return {
+        default: {
+          abi: MODULE_UI_ABI_VERSION,
+          id: "announcements",
+          version: "1.2.0",
+          mount: () => undefined,
+        },
+      };
     };
 
     const mounted = await loadModuleUi({
diff --git a/packages/module-ui-host/tests/real-artifact.test.ts b/packages/module-ui-host/tests/real-artifact.test.ts
index fdba0f3..4888ab1 100644
--- a/packages/module-ui-host/tests/real-artifact.test.ts
+++ b/packages/module-ui-host/tests/real-artifact.test.ts
@@ -5,7 +5,20 @@ import { fileURLToPath } from "node:url";
 import { buildModuleUi } from "@ekklesia/module-ui-build";
 import type { ModuleUiHostContext } from "@ekklesia/module-ui-sdk";
 import { beforeAll, describe, expect, it } from "vitest";
-import { loadModuleUi } from "../src/index.js";
+import {
+  loadModuleUi,
+  MODULE_UI_ISOLATION_ATTRIBUTE,
+  MODULE_UI_ISOLATION_ROOT_ATTRIBUTE,
+} from "../src/index.js";
+
+/** What the artifact rendered, reached through the open shadow root the loader created (ADR 0040). */
+function artifactText(container: Element): string {
+  const shadowHost = container.querySelector(`[${MODULE_UI_ISOLATION_ATTRIBUTE}]`);
+  return (
+    shadowHost?.shadowRoot?.querySelector(`[${MODULE_UI_ISOLATION_ROOT_ATTRIBUTE}]`)?.textContent ??
+    ""
+  );
+}
 
 const fixtureDirectory = path.join(
   path.dirname(fileURLToPath(import.meta.url)),
@@ -80,12 +93,32 @@ describe("loading a real compiled artifact", () => {
       context: ekklesiaContext(requested),
     });
 
-    expect(container.textContent).toBe("example:200:user-123");
+    expect(artifactText(container)).toBe("example:200:user-123");
     await mounted.unmount();
-    expect(container.textContent).toBe("");
+    expect(container.children).toHaveLength(0);
     expect(mounted.mounted).toBe(false);
   });
 
+  it("compiles the fixture's stylesheet into the artifact and installs it in the isolated root", async () => {
+    const container = document.createElement("div");
+
+    // No sidecar: the whole stylesheet travels inside the one declared JavaScript artifact.
+    expect(artifactSource).toContain(".artifact-label");
+
+    const mounted = await loadModuleUi({
+      source: { moduleId: "example", version: "0.1.0", url: artifactUrl },
+      container,
+      context: ekklesiaContext([]),
+    });
+
+    const stylesheet = mounted.shadowRoot.querySelector("style")?.textContent ?? "";
+    expect(stylesheet).toContain(".artifact-label");
+    expect(stylesheet).toContain("var(--ekklesia-text-primary)");
+    expect(document.head.textContent ?? "").not.toContain(".artifact-label");
+
+    await mounted.unmount();
+  });
+
   it("still rejects the real artifact when the host expects a different version", async () => {
     await expect(
       loadModuleUi({
@@ -118,8 +151,8 @@ describe("loading a real compiled artifact", () => {
     // the module asked for "records" both times and resolved nothing itself.
     expect(ekklesiaRequests).toEqual(["/api/v1/modules/example/records"]);
     expect(standaloneRequests).toEqual(["https://example.test/api/records"]);
-    expect(ekklesiaContainer.textContent).toBe("example:200:user-123");
-    expect(standaloneContainer.textContent).toBe("example:200:guest");
+    expect(artifactText(ekklesiaContainer)).toBe("example:200:user-123");
+    expect(artifactText(standaloneContainer)).toBe("example:200:guest");
 
     await inEkklesia.unmount();
     await inStandalone.unmount();
diff --git a/packages/module-ui-sdk/src/define-module-ui.ts b/packages/module-ui-sdk/src/define-module-ui.ts
index 263e5cb..8668459 100644
--- a/packages/module-ui-sdk/src/define-module-ui.ts
+++ b/packages/module-ui-sdk/src/define-module-ui.ts
@@ -1,12 +1,18 @@
-import { type ModuleUiDefinition, parseModuleUiDefinition } from "./module-ui.js";
+import {
+  MODULE_UI_ABI_VERSION,
+  type ModuleUiDefinition,
+  type ModuleUiDefinitionInput,
+  parseModuleUiDefinition,
+} from "./module-ui.js";
 
 /**
  * The module author's entry point, mirroring `defineModuleRuntime` (ADR 0006): validate at authoring
- * time, freeze, and preserve the literal type. A malformed definition fails where it was written
- * rather than in a host, at mount time, in a browser.
+ * time, freeze, and stamp the browser mount ABI this artifact was built against. A malformed
+ * definition fails where it was written rather than in a host, at mount time, in a browser.
+ *
+ * The ABI marker is added here rather than written by the author, so it can never drift from the SDK
+ * the artifact actually compiled against — which is the whole point of having one (ADR 0040).
  */
-export function defineModuleUi<const Definition extends ModuleUiDefinition>(
-  definition: Definition,
-): Readonly<Definition> {
-  return parseModuleUiDefinition(definition) as Readonly<Definition>;
+export function defineModuleUi(definition: ModuleUiDefinitionInput): ModuleUiDefinition {
+  return parseModuleUiDefinition({ ...definition, abi: MODULE_UI_ABI_VERSION });
 }
diff --git a/packages/module-ui-sdk/src/module-ui.ts b/packages/module-ui-sdk/src/module-ui.ts
index 9ae8452..cd60aec 100644
--- a/packages/module-ui-sdk/src/module-ui.ts
+++ b/packages/module-ui-sdk/src/module-ui.ts
@@ -2,17 +2,38 @@ import type { ModuleUiHostContext } from "./host-context.js";
 
 export type Awaitable<Value> = Value | PromiseLike<Value>;
 
+/**
+ * The version of the browser mount ABI this package defines, and the only one a host accepts.
+ *
+ * Version 1 was `{ id, version, mount }` mounted straight into the host's own container. Version 2
+ * adds a declared `styles` stylesheet and makes every artifact mount inside an **isolated root** the
+ * host creates (ADR 0040). That is a behavioural change an artifact can observe — its DOM is no
+ * longer a direct child of the host's element — so it is versioned explicitly rather than
+ * introduced silently.
+ *
+ * There is deliberately no dual acceptance, exactly as there is none for a manifest version
+ * (ADR 0006, ADR 0014, ADR 0021, ADR 0033): an artifact built against version 1 fails to mount with
+ * `MODULE_UI_ABI_UNSUPPORTED` rather than being mounted under guessed semantics. Migration is to
+ * rebuild the module, which is one `pnpm build` — the canonical builder owns every option, and a
+ * module that declares no stylesheet needs no source change at all.
+ *
+ * `defineModuleUi` stamps this for the author. A module never writes it.
+ */
+export const MODULE_UI_ABI_VERSION = 2;
+
 /**
  * Released by a host when the module UI should be torn down. A module that acquires anything during
  * `mount` — a framework root, a subscription, a timer, an event listener — must return one of these
  * and release that work here, exactly as a backend module releases resources in its `stop` hook
  * (ADR 0006). Returning nothing declares that the mount acquired nothing to release.
+ *
+ * A module never has to remove its declared `styles`: the host installed them into the isolated root
+ * it owns, and it removes that whole root after this runs.
  */
 export type ModuleUiUnmount = () => Awaitable<void>;
 
 /**
- * What either compiled browser artifact (`entrypoints.ui` or `entrypoints.display.artifact`)
- * default-exports (ADR 0021, ADR 0022, ADR 0033).
+ * What a module author passes to {@link defineModuleUi}.
  *
  * `id` and `version` are build-artifact identity markers, mirroring `ModuleRuntimeDefinition`
  * exactly (ADR 0006): they let a host prove the UI bundle it imported was built from the module it
@@ -23,19 +44,38 @@ export type ModuleUiUnmount = () => Awaitable<void>;
  * write that (ADR 0022), but React appears nowhere in this type — the contract is a mount function,
  * so a module's internal framework choice, and a host's, can each change without renegotiating the
  * platform boundary.
+ *
+ * `styles` is the artifact's own compiled CSS **text**, not a URL, a filename, or a sidecar
+ * (ADR 0040). A module imports its local `.css` files and the canonical builder compiles them into
+ * this string, so the declared JavaScript artifact stays exactly as self-contained as it was before
+ * stylesheets existed: there is no second file to publish, nothing for a host to fetch, and no way
+ * for a stylesheet to become an asset nothing serves. The host installs it into the artifact's
+ * isolated root at mount and removes it at unmount.
  */
-export interface ModuleUiDefinition {
+export interface ModuleUiDefinitionInput {
   readonly id: string;
   readonly version: string;
+  readonly styles?: string;
   readonly mount: (
     container: Element,
     context: ModuleUiHostContext,
   ) => Awaitable<ModuleUiUnmount | void>;
 }
 
+/**
+ * What either compiled browser artifact (`entrypoints.ui` or `entrypoints.display.artifact`)
+ * default-exports (ADR 0021, ADR 0022, ADR 0033, ADR 0040): the author's declaration plus the ABI
+ * marker `defineModuleUi` stamped on it.
+ */
+export interface ModuleUiDefinition extends ModuleUiDefinitionInput {
+  readonly abi: number;
+}
+
 export type ModuleUiContractErrorCode =
   | "MODULE_UI_EXPORT_MISSING"
   | "MODULE_UI_DEFINITION_INVALID"
+  | "MODULE_UI_ABI_UNSUPPORTED"
+  | "MODULE_UI_STYLES_INVALID"
   | "MODULE_UI_IDENTITY_MISMATCH";
 
 const errorMessages: Readonly<Record<ModuleUiContractErrorCode, string>> = Object.freeze({
@@ -43,6 +83,10 @@ const errorMessages: Readonly<Record<ModuleUiContractErrorCode, string>> = Objec
     "A module UI entrypoint must have a default export produced by defineModuleUi.",
   MODULE_UI_DEFINITION_INVALID:
     "A module UI definition must declare a non-empty id and version and a mount function, and nothing else.",
+  MODULE_UI_ABI_UNSUPPORTED:
+    "This module UI artifact was built for a different browser mount ABI. Rebuild the module against the current module UI SDK.",
+  MODULE_UI_STYLES_INVALID:
+    "A module UI definition's styles must be the artifact's compiled CSS text, as a non-empty string.",
   MODULE_UI_IDENTITY_MISMATCH:
     "The module UI artifact's id and version do not match the module it was loaded for.",
 });
@@ -64,7 +108,7 @@ export class ModuleUiContractError extends Error {
   }
 }
 
-const definitionKeys = new Set(["id", "mount", "version"]);
+const definitionKeys = new Set(["abi", "id", "mount", "styles", "version"]);
 
 function isRecord(value: unknown): value is Record<string, unknown> {
   return typeof value === "object" && value !== null && !Array.isArray(value);
@@ -80,12 +124,15 @@ function isNonEmptyString(value: unknown): value is string {
  * grammar — there is exactly one place each of those is defined, and it is not here.
  */
 export function isModuleUiDefinition(value: unknown): value is ModuleUiDefinition {
-  if (!isRecord(value)) {
+  if (!isRecord(value) || value.abi !== MODULE_UI_ABI_VERSION) {
     return false;
   }
   if (Object.keys(value).some((key) => !definitionKeys.has(key))) {
     return false;
   }
+  if (value.styles !== undefined && !isNonEmptyString(value.styles)) {
+    return false;
+  }
   return (
     isNonEmptyString(value.id) &&
     isNonEmptyString(value.version) &&
@@ -93,13 +140,31 @@ export function isModuleUiDefinition(value: unknown): value is ModuleUiDefinitio
   );
 }
 
+/**
+ * Parses an artifact's default export.
+ *
+ * The ABI marker is checked **first and on its own**, so an artifact built against the previous
+ * mount ABI is reported as exactly that rather than as a malformed definition — the difference
+ * between "rebuild your module" and a developer hunting for a typo.
+ */
 export function parseModuleUiDefinition(value: unknown): ModuleUiDefinition {
+  if (!isRecord(value)) {
+    throw new ModuleUiContractError("MODULE_UI_DEFINITION_INVALID");
+  }
+  if (value.abi !== MODULE_UI_ABI_VERSION) {
+    throw new ModuleUiContractError("MODULE_UI_ABI_UNSUPPORTED");
+  }
+  if (value.styles !== undefined && !isNonEmptyString(value.styles)) {
+    throw new ModuleUiContractError("MODULE_UI_STYLES_INVALID");
+  }
   if (!isModuleUiDefinition(value)) {
     throw new ModuleUiContractError("MODULE_UI_DEFINITION_INVALID");
   }
   return Object.freeze({
+    abi: MODULE_UI_ABI_VERSION,
     id: value.id,
     mount: value.mount,
+    ...(value.styles === undefined ? {} : { styles: value.styles }),
     version: value.version,
   });
 }
diff --git a/packages/module-ui-sdk/tests/module-ui-contract.test.ts b/packages/module-ui-sdk/tests/module-ui-contract.test.ts
index 3e3d28e..5428d10 100644
--- a/packages/module-ui-sdk/tests/module-ui-contract.test.ts
+++ b/packages/module-ui-sdk/tests/module-ui-contract.test.ts
@@ -2,23 +2,32 @@ import { beforeEach, describe, expect, it } from "vitest";
 import {
   defineModuleUi,
   isModuleUiDefinition,
+  MODULE_UI_ABI_VERSION,
   ModuleUiContractError,
   type ModuleUiApiClient,
   type ModuleUiDefinition,
+  type ModuleUiDefinitionInput,
   type ModuleUiHostContext,
   type ModuleUiRealtimeClient,
   parseModuleUiDefinition,
   parseModuleUiExport,
 } from "../src/index.js";
 
-function validDefinition(): ModuleUiDefinition {
+function definitionInput(): ModuleUiDefinitionInput {
   return { id: "announcements", version: "0.1.0", mount: (): void => undefined };
 }
 
+/** What a compiled artifact actually default-exports: the author's declaration plus the ABI marker
+ * `defineModuleUi` stamps on it (ADR 0040). */
+function validDefinition(): ModuleUiDefinition {
+  return { abi: MODULE_UI_ABI_VERSION, ...definitionInput() };
+}
+
 /** The contract treats the container as an opaque host-supplied mount target, so a plain object
  * with the one property the fixture touches is a faithful stand-in. No DOM is required to prove the
  * contract, and this package deliberately does not take a DOM dependency to obtain one. */
 const noop = (): void => undefined;
+const abi = MODULE_UI_ABI_VERSION;
 
 const noopRealtime: ModuleUiRealtimeClient = Object.freeze({
   subscribe: () => Object.freeze({ close: noop }),
@@ -29,23 +38,66 @@ function fakeContainer(): Element & { textContent: string } {
 }
 
 describe("module UI definition contract", () => {
-  it("accepts and freezes a valid definition", () => {
-    const definition = defineModuleUi(validDefinition());
+  it("accepts, stamps the ABI on, and freezes a valid definition", () => {
+    const definition = defineModuleUi(definitionInput());
 
     expect(definition.id).toBe("announcements");
     expect(definition.version).toBe("0.1.0");
+    // Stamped by the SDK the artifact compiled against, never written by the author, so it can
+    // never drift from the contract the artifact was actually built for.
+    expect(definition.abi).toBe(MODULE_UI_ABI_VERSION);
+    expect(definition.styles).toBeUndefined();
     expect(Object.isFrozen(definition)).toBe(true);
   });
 
+  it("carries a declared stylesheet through as the artifact's own compiled CSS text", () => {
+    const definition = defineModuleUi({ ...definitionInput(), styles: ".a{color:red}" });
+
+    expect(definition.styles).toBe(".a{color:red}");
+  });
+
+  it("rejects an artifact built against a different mount ABI, with its own code", () => {
+    // The whole point of the marker: a version-1 artifact is reported as "rebuild your module",
+    // not as a malformed definition a developer would hunt for a typo in.
+    const versionOne = { id: "announcements", version: "0.1.0", mount: noop };
+
+    expect(isModuleUiDefinition(versionOne)).toBe(false);
+    try {
+      parseModuleUiDefinition(versionOne);
+      expect.unreachable("expected a contract error");
+    } catch (error) {
+      expect((error as ModuleUiContractError).code).toBe("MODULE_UI_ABI_UNSUPPORTED");
+    }
+    expect(() =>
+      parseModuleUiDefinition({ ...versionOne, abi: MODULE_UI_ABI_VERSION + 1 }),
+    ).toThrowError(expect.objectContaining({ code: "MODULE_UI_ABI_UNSUPPORTED" }));
+  });
+
   it.each<[string, unknown]>([
-    ["a missing id", { version: "0.1.0", mount: noop }],
-    ["a blank id", { id: "   ", version: "0.1.0", mount: noop }],
-    ["a non-string id", { id: 7, version: "0.1.0", mount: noop }],
-    ["a missing version", { id: "announcements", mount: noop }],
-    ["a blank version", { id: "announcements", version: "", mount: noop }],
-    ["a missing mount", { id: "announcements", version: "0.1.0" }],
-    ["a non-function mount", { id: "announcements", version: "0.1.0", mount: "render" }],
-    ["an unknown property", { id: "announcements", version: "0.1.0", mount: noop, routes: [] }],
+    ["a blank stylesheet", "   "],
+    ["a non-string stylesheet", 7],
+    ["an array of stylesheets", [".a{}"]],
+  ])("rejects %s", (_label, styles) => {
+    const candidate = { ...validDefinition(), styles };
+
+    expect(isModuleUiDefinition(candidate)).toBe(false);
+    expect(() => parseModuleUiDefinition(candidate)).toThrowError(
+      expect.objectContaining({ code: "MODULE_UI_STYLES_INVALID" }),
+    );
+  });
+
+  it.each<[string, unknown]>([
+    ["a missing id", { abi, version: "0.1.0", mount: noop }],
+    ["a blank id", { abi, id: "   ", version: "0.1.0", mount: noop }],
+    ["a non-string id", { abi, id: 7, version: "0.1.0", mount: noop }],
+    ["a missing version", { abi, id: "announcements", mount: noop }],
+    ["a blank version", { abi, id: "announcements", version: "", mount: noop }],
+    ["a missing mount", { abi, id: "announcements", version: "0.1.0" }],
+    ["a non-function mount", { abi, id: "announcements", version: "0.1.0", mount: "render" }],
+    [
+      "an unknown property",
+      { abi, id: "announcements", version: "0.1.0", mount: noop, routes: [] },
+    ],
   ])("rejects a definition with %s", (_label, candidate) => {
     expect(isModuleUiDefinition(candidate)).toBe(false);
     expect(() => parseModuleUiDefinition(candidate)).toThrowError(ModuleUiContractError);
@@ -64,7 +116,7 @@ describe("module UI definition contract", () => {
 
   it("reports a malformed definition with a stable, sanitized code", () => {
     try {
-      parseModuleUiDefinition({ id: "announcements" });
+      parseModuleUiDefinition({ abi, id: "announcements" });
       expect.unreachable("expected a contract error");
     } catch (error) {
       expect(error).toBeInstanceOf(ModuleUiContractError);
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 0c7db4c..469eda1 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -58,6 +58,9 @@ importers:
       '@ekklesia/module-ui-host':
         specifier: workspace:*
         version: link:../../packages/module-ui-host
+      '@ekklesia/module-ui-sdk':
+        specifier: workspace:*
+        version: link:../../packages/module-ui-sdk
       '@ekklesia/typescript-config':
         specifier: workspace:*
         version: link:../../packages/typescript-config
@@ -73,6 +76,9 @@ importers:
       better-auth:
         specifier: 1.6.23
         version: 1.6.23(drizzle-kit@0.31.10)(drizzle-orm@0.45.2(@types/pg@8.20.0)(kysely@0.29.4)(pg@8.22.0))(pg@8.22.0)(react-dom@19.2.8(react@19.2.8))(react@19.2.8)(vitest@4.1.10(@types/node@24.13.3)(happy-dom@20.0.11)(vite@8.1.4(@types/node@24.13.3)(esbuild@0.28.1)(jiti@2.7.0)(tsx@4.23.1)(yaml@2.9.0)))
+      happy-dom:
+        specifier: 20.0.11
+        version: 20.0.11
       pg:
         specifier: 8.22.0
         version: 8.22.0
@@ -364,12 +370,18 @@ importers:
 
   templates/module:
     dependencies:
+      '@ekklesia/design-tokens':
+        specifier: workspace:*
+        version: link:../../packages/design-tokens
       '@ekklesia/module-sdk':
         specifier: workspace:*
         version: link:../../packages/module-sdk
       '@ekklesia/module-ui-sdk':
         specifier: workspace:*
         version: link:../../packages/module-ui-sdk
+      '@ekklesia/ui':
+        specifier: workspace:*
+        version: link:../../packages/ui
       drizzle-orm:
         specifier: 0.45.2
         version: 0.45.2(@types/pg@8.20.0)(kysely@0.29.4)(pg@8.22.0)
diff --git a/scripts/check-boundaries.ts b/scripts/check-boundaries.ts
index f01447d..5811b26 100644
--- a/scripts/check-boundaries.ts
+++ b/scripts/check-boundaries.ts
@@ -49,12 +49,13 @@ export interface BoundaryResult {
 const CODE_EXTENSIONS = new Set([".cjs", ".cts", ".js", ".jsx", ".mjs", ".mts", ".ts", ".tsx"]);
 // Presentation formats a module may NOT contain. `.tsx` is deliberately absent: React is the
 // supported module UI authoring model (ADR 0022), so a module's own `.tsx` is legitimate source.
-// `.jsx` stays rejected because this repository is strict-TypeScript throughout, and `.css` stays
-// rejected because module styling and the module UI build pipeline are not designed yet — allowing
-// a stylesheet would imply a bundling contract that does not exist.
+// `.jsx` stays rejected because this repository is strict-TypeScript throughout. `.css` is handled
+// separately below rather than listed here: since ADR 0040 a module may author a plain stylesheet,
+// but only inside the browser-artifact source roots the canonical builder actually compiles. Every
+// other stylesheet dialect stays rejected outright — adding one is a toolchain decision needing its
+// own ADR, not a detail of a build pass.
 const MODULE_UI_EXTENSIONS = new Set([
   ".astro",
-  ".css",
   ".htm",
   ".html",
   ".jsx",
@@ -161,6 +162,25 @@ const SHARED_PACKAGE_ROLES = new Set([
   "ui",
 ]);
 
+/**
+ * The module-package-relative directories whose contents `@ekklesia/module-ui-build` compiles into a
+ * declared browser artifact (ADR 0023, ADR 0033, ADR 0040).
+ *
+ * A stylesheet is only meaningful inside one of these: anywhere else in a module it would be a file
+ * no build reads, no manifest declares, and no host ever loads — the undeclared-sidecar failure this
+ * repository has rejected since ADR 0023, just relocated. Restricting the location is also what
+ * keeps the relaxation honest: a module gains stylesheets for its browser artifacts, not the ability
+ * to scatter presentation files through its backend.
+ */
+const MODULE_BROWSER_SOURCE_ROOTS = ["src/ui/", "src/display/"];
+
+/** Whether a repository-relative path inside a module package sits in one of those roots. Module
+ * packages live at `modules/<name>/…` and `templates/module/…`, both two segments deep. */
+function isModuleBrowserSource(relativePath: string): boolean {
+  const withinPackage = relativePath.split("/").slice(2).join("/");
+  return MODULE_BROWSER_SOURCE_ROOTS.some((sourceRoot) => withinPackage.startsWith(sourceRoot));
+}
+
 function normalizePath(value: string): string {
   return value.split(path.sep).join("/").replace(/^\.\//, "");
 }
@@ -856,10 +876,21 @@ export async function checkBoundaries(rootDirectory: string): Promise<BoundaryRe
   for (const repositoryFile of allFiles) {
     const relativeFile = normalizePath(path.relative(root, repositoryFile));
     const location = describeRepositoryPath(relativeFile);
-    if (
-      location.layer === "module" &&
-      MODULE_UI_EXTENSIONS.has(path.extname(repositoryFile).toLowerCase())
-    ) {
+    if (location.layer !== "module") {
+      continue;
+    }
+    const extension = path.extname(repositoryFile).toLowerCase();
+    if (extension === ".css") {
+      if (!isModuleBrowserSource(location.relativePath)) {
+        violations.push({
+          from: relativeFile,
+          message: `A module stylesheet must live inside a compiled browser source root (${MODULE_BROWSER_SOURCE_ROOTS.join(", ")}).`,
+          rule: "modules-do-not-own-ui",
+        });
+      }
+      continue;
+    }
+    if (MODULE_UI_EXTENSIONS.has(extension)) {
       violations.push({
         from: relativeFile,
         message: "UI source files belong in a surface, not a module.",
diff --git a/surfaces/shell/tests/display-app.test.tsx b/surfaces/shell/tests/display-app.test.tsx
index 35aa48d..01552dd 100644
--- a/surfaces/shell/tests/display-app.test.tsx
+++ b/surfaces/shell/tests/display-app.test.tsx
@@ -1,4 +1,4 @@
-import type { ModuleUiHostContext } from "@ekklesia/module-ui-sdk";
+import { MODULE_UI_ABI_VERSION, type ModuleUiHostContext } from "@ekklesia/module-ui-sdk";
 import { act, Component, type ReactElement, type ReactNode } from "react";
 import { createRoot, type Root } from "react-dom/client";
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
@@ -9,6 +9,7 @@ import type {
   HostEventSourceFactory,
   HostEventSourceOptions,
 } from "../src/host-protocol.js";
+import { deepQuery, isolatedMountTarget, isolatedText } from "./support/isolated-dom.js";
 
 declare global {
   // eslint-disable-next-line no-var
@@ -154,6 +155,7 @@ function artifact(
 ) {
   return {
     default: {
+      abi: MODULE_UI_ABI_VERSION,
       id,
       mount(element: Element, context: ModuleUiHostContext) {
         if (options.mount) {
@@ -204,7 +206,7 @@ describe("display host — full-viewport boundary", () => {
     });
 
     expect(imported).toEqual([`${apiBaseUrl}/api/v1/module-ui/announcements/1.2.0/display`]);
-    expect(query("display-module-container")?.textContent).toBe("announcements:display");
+    expect(isolatedText(query("display-module-container"))).toBe("announcements:display");
     expect(container.querySelector("header")).toBeNull();
     expect(container.querySelector("nav")).toBeNull();
     expect(container.querySelector("aside")).toBeNull();
@@ -239,7 +241,7 @@ describe("display host — full-viewport boundary", () => {
       "realtime",
     ]);
     expect(seenContext?.principal).toEqual({ kind: "authenticated", userId: "user-1" });
-    expect(query("display-module-container")?.textContent).toBe("permitted");
+    expect(isolatedText(query("display-module-container"))).toBe("permitted");
     expect(
       server.calls.some((call) => call.url.endsWith("/api/v1/modules/announcements/slides")),
     ).toBe(true);
@@ -255,6 +257,7 @@ describe("display host — full-viewport boundary", () => {
     const requestedArtifacts: string[] = [];
     const source = `
       export default {
+        abi: ${MODULE_UI_ABI_VERSION},
         id: "announcements",
         version: "1.2.0",
         mount(container, context) {
@@ -303,7 +306,7 @@ describe("display host — full-viewport boundary", () => {
     expect(requestedArtifacts).toEqual([
       `${apiBaseUrl}/api/v1/module-ui/announcements/1.2.0/display`,
     ]);
-    expect(container.querySelector('[data-testid="announcement-slide"]')?.getAttribute("src")).toBe(
+    expect(deepQuery(container, '[data-testid="announcement-slide"]')?.getAttribute("src")).toBe(
       deliveryUrl,
     );
     expect(container.innerHTML).not.toContain("socket.io");
@@ -375,7 +378,10 @@ describe("display host — full-viewport boundary", () => {
     };
 
     await act(async () => root.render(<HostApp config={config} importModule={importer} />));
-    expect(container.textContent).toContain("announcement management");
+    // The management artifact rendered inside its own isolated root, not the shell's own tree.
+    expect(isolatedText(container.querySelector('[data-testid="module-container"]'))).toContain(
+      "announcement management",
+    );
     expect(container.querySelector("nav")).not.toBeNull();
     expect(requestedArtifacts).toEqual([`${apiBaseUrl}/api/v1/module-ui/announcements/1.2.0`]);
 
@@ -401,7 +407,7 @@ describe("display host — full-viewport boundary", () => {
       `${apiBaseUrl}/api/v1/module-ui/announcements/1.2.0`,
       `${apiBaseUrl}/api/v1/module-ui/announcements/1.2.0/display`,
     ]);
-    expect(container.querySelector('[data-testid="both-role-slide"]')?.getAttribute("src")).toBe(
+    expect(deepQuery(container, '[data-testid="both-role-slide"]')?.getAttribute("src")).toBe(
       deliveryUrl,
     );
     expect(container.querySelector("nav")).toBeNull();
@@ -478,7 +484,7 @@ describe("display host — authentication and controlled failures", () => {
     await act(async () => query("display-retry")?.click());
 
     expect(query("display-error")).toBeNull();
-    expect(query("display-module-container")?.textContent).toBe("recovered");
+    expect(isolatedText(query("display-module-container"))).toBe("recovered");
     expect(containers).toHaveLength(2);
     expect(containers[0]).not.toBe(containers[1]);
     expect(containers[0]?.isConnected).toBe(false);
@@ -508,7 +514,7 @@ describe("display host — authentication and controlled failures", () => {
     );
 
     await render({ apiBaseUrl, fetch: server.fetch }, importer);
-    expect(query("display-module-container")?.textContent).toContain("/srv/modules");
+    expect(isolatedText(query("display-module-container"))).toContain("/srv/modules");
     consoleError.mockClear();
 
     await act(async () => {
@@ -525,7 +531,7 @@ describe("display host — authentication and controlled failures", () => {
     });
 
     expect(query("display-error")).not.toBeNull();
-    expect(query("display-module-container")?.textContent).toBe("");
+    expect(isolatedText(query("display-module-container"))).toBe("");
     expect(container.textContent).toBe("This display could not be loaded.Retry");
     expect(container.textContent).not.toMatch(/secret|\/srv\//u);
     expect(importer).toHaveBeenCalledTimes(1);
@@ -541,7 +547,7 @@ describe("display host — authentication and controlled failures", () => {
 
     expect(importer).toHaveBeenCalledTimes(2);
     expect(query("display-error")).toBeNull();
-    expect(query("display-module-container")?.textContent).toContain("/srv/modules");
+    expect(isolatedText(query("display-module-container"))).toContain("/srv/modules");
     expect(mountedInto[0]?.isConnected).toBe(false);
     expect(mountedInto[1]?.isConnected).toBe(true);
   });
@@ -610,7 +616,7 @@ describe("display host — authentication and controlled failures", () => {
     );
 
     await act(async () => {
-      container.querySelector('[data-testid="display-media"]')?.dispatchEvent(
+      deepQuery(container, '[data-testid="display-media"]')?.dispatchEvent(
         new ErrorEvent("error", {
           bubbles: true,
           cancelable: true,
@@ -620,7 +626,7 @@ describe("display host — authentication and controlled failures", () => {
     });
 
     expect(query("display-error")).toBeNull();
-    expect(query("display-media")).not.toBeNull();
+    expect(deepQuery(container, '[data-testid="display-media"]')).not.toBeNull();
     expect(cleanup).not.toHaveBeenCalled();
   });
 
@@ -686,7 +692,7 @@ describe("display host — independent instances and stale work", () => {
 
     expect(cleanups).toHaveLength(1);
     expect(container.querySelectorAll('[data-testid="display-module-container"]')).toHaveLength(1);
-    expect(query("display-module-container")?.textContent).toBe("announcements:display");
+    expect(isolatedText(query("display-module-container"))).toBe("announcements:display");
   });
 
   it("discards a late import into its detached container without disturbing the newer route", async () => {
@@ -728,7 +734,7 @@ describe("display host — independent instances and stale work", () => {
 
     await render(config, importer);
     await render(config, importer, "calendar");
-    expect(query("display-module-container")?.textContent).toBe("calendar-live");
+    expect(isolatedText(query("display-module-container"))).toBe("calendar-live");
     const liveContainer = query("display-module-container");
 
     await act(async () => {
@@ -737,8 +743,9 @@ describe("display host — independent instances and stale work", () => {
     });
 
     expect(query("display-module-container")).toBe(liveContainer);
-    expect(liveContainer?.textContent).toBe("calendar-live");
-    const staleContainer = mountedInto.find((element) => element !== liveContainer);
+    expect(isolatedText(liveContainer)).toBe("calendar-live");
+    const liveMountTarget = isolatedMountTarget(liveContainer);
+    const staleContainer = mountedInto.find((element) => element !== liveMountTarget);
     expect(staleContainer?.isConnected).toBe(false);
     expect(staleUnmounts).toEqual([staleContainer]);
   });
diff --git a/surfaces/shell/tests/host-app.test.tsx b/surfaces/shell/tests/host-app.test.tsx
index c78d64c..850bb86 100644
--- a/surfaces/shell/tests/host-app.test.tsx
+++ b/surfaces/shell/tests/host-app.test.tsx
@@ -1,9 +1,10 @@
-import type { ModuleUiHostContext } from "@ekklesia/module-ui-sdk";
+import { MODULE_UI_ABI_VERSION, type ModuleUiHostContext } from "@ekklesia/module-ui-sdk";
 import { act } from "react";
 import { createRoot, type Root } from "react-dom/client";
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import { HostApp } from "../src/browser/HostApp.js";
 import type { EkklesiaHostConfig } from "../src/host-protocol.js";
+import { isolatedMountTarget, isolatedText } from "./support/isolated-dom.js";
 
 declare global {
   // eslint-disable-next-line no-var
@@ -99,6 +100,7 @@ interface ArtifactOptions {
 function artifact(id: string, version: string, options: ArtifactOptions = {}) {
   return {
     default: {
+      abi: MODULE_UI_ABI_VERSION,
       id,
       mount(element: Element, context: ModuleUiHostContext) {
         options.mountedInto?.push(element);
@@ -443,7 +445,7 @@ describe("Ekklesia shell — browser authentication", () => {
 
     expect(query("host-login")).toBeNull();
     expect(query("host-logout")).not.toBeNull();
-    expect(query("module-container")?.textContent).toBe("example mounted");
+    expect(isolatedText(query("module-container"))).toBe("example mounted");
     expect(importer).toHaveBeenCalledOnce();
     expect(platform.calls.map((call) => new URL(call.url).pathname)).toEqual([
       "/api/v1/principal",
@@ -565,6 +567,7 @@ describe("Ekklesia shell — logout and session expiration", () => {
       },
       async () => ({
         default: {
+          abi: MODULE_UI_ABI_VERSION,
           id: "example",
           mount(element: Element, context: ModuleUiHostContext) {
             element.textContent = "example mounted";
@@ -693,6 +696,7 @@ describe("Ekklesia shell — logout and session expiration", () => {
   ) {
     return {
       default: {
+        abi: MODULE_UI_ABI_VERSION,
         id: "example",
         mount(element: Element, context: ModuleUiHostContext) {
           capture.context = context;
@@ -770,7 +774,7 @@ describe("Ekklesia shell — logout and session expiration", () => {
     });
 
     expect(query("host-login")).toBeNull();
-    expect(query("module-container")?.textContent).toBe("example mounted");
+    expect(isolatedText(query("module-container"))).toBe("example mounted");
     expect(capture.unmountCalls).toBe(0);
     expect(platform.calls.filter((call) => call.url.endsWith("/api/v1/principal"))).toHaveLength(2);
   });
@@ -790,7 +794,7 @@ describe("Ekklesia shell — logout and session expiration", () => {
     });
 
     expect(query("host-login")).toBeNull();
-    expect(query("module-container")?.textContent).toBe("example mounted");
+    expect(isolatedText(query("module-container"))).toBe("example mounted");
     expect(capture.unmountCalls).toBe(0);
     expect(platform.calls.filter((call) => call.url.endsWith("/api/v1/principal"))).toHaveLength(2);
   });
@@ -920,7 +924,7 @@ describe("Ekklesia shell — Dashboard navigation", () => {
       `${apiBaseUrl}/api/v1/module-ui/video/0.1.0`,
       `${apiBaseUrl}/api/v1/module-ui/calendar/0.1.0`,
     ]);
-    expect(query("module-container")?.textContent).toBe("calendar mounted");
+    expect(isolatedText(query("module-container"))).toBe("calendar mounted");
   });
 
   it("mounts the identical artifact whichever group a module is presented in", async () => {
@@ -972,10 +976,11 @@ describe("Ekklesia shell — mount lifecycle", () => {
       artifact("example", "0.1.0", { mountedInto }),
     );
 
-    expect(query("module-container")?.textContent).toBe("example mounted");
+    expect(isolatedText(query("module-container"))).toBe("example mounted");
     expect(query("module-loading")).toBeNull();
     // The module was handed the shell's container, never the page root or the shell's own layout.
-    expect(mountedInto[0]).toBe(query("module-container"));
+    // The artifact receives the loader's isolated root, never the shell's own element (ADR 0040).
+    expect(mountedInto[0]).toBe(isolatedMountTarget(query("module-container")));
     expect(mountedInto[0]).not.toBe(container);
     expect(mountedInto[0]).not.toBe(document.body);
   });
@@ -1002,7 +1007,7 @@ describe("Ekklesia shell — mount lifecycle", () => {
     });
 
     expect(query("module-loading")).not.toBeNull();
-    expect(query("module-container")?.textContent).toBe("");
+    expect(isolatedText(query("module-container"))).toBe("");
 
     await act(async () => {
       release?.();
@@ -1010,7 +1015,7 @@ describe("Ekklesia shell — mount lifecycle", () => {
     });
 
     expect(query("module-loading")).toBeNull();
-    expect(query("module-container")?.textContent).toBe("example mounted");
+    expect(isolatedText(query("module-container"))).toBe("example mounted");
   });
 
   it("unmounts the previous module when the selection changes", async () => {
@@ -1019,6 +1024,7 @@ describe("Ekklesia shell — mount lifecycle", () => {
       const id = url.includes("/calendar/") ? "calendar" : "example";
       return {
         default: {
+          abi: MODULE_UI_ABI_VERSION,
           id,
           mount(element: Element) {
             element.textContent = `${id} mounted`;
@@ -1040,14 +1046,14 @@ describe("Ekklesia shell — mount lifecycle", () => {
       },
       importModule,
     );
-    expect(query("module-container")?.textContent).toBe("example mounted");
+    expect(isolatedText(query("module-container"))).toBe("example mounted");
 
     await act(async () => {
       (query("select-calendar") as HTMLButtonElement).click();
     });
 
     expect(unmountedContainers).toHaveLength(1);
-    expect(query("module-container")?.textContent).toBe("calendar mounted");
+    expect(isolatedText(query("module-container"))).toBe("calendar mounted");
   });
 
   it("keeps a stale load from ever reaching the newer selection's container", async () => {
@@ -1065,6 +1071,7 @@ describe("Ekklesia shell — mount lifecycle", () => {
       }
       return {
         default: {
+          abi: MODULE_UI_ABI_VERSION,
           id,
           mount(element: Element) {
             mountedInto.push(element);
@@ -1096,7 +1103,8 @@ describe("Ekklesia shell — mount lifecycle", () => {
       (query("select-calendar") as HTMLButtonElement).click();
     });
     const liveContainer = query("module-container");
-    expect(liveContainer?.textContent).toBe("calendar mounted");
+    const liveMountTarget = isolatedMountTarget(liveContainer);
+    expect(isolatedText(liveContainer)).toBe("calendar mounted");
 
     // Only now does the stale `example` import resolve.
     await act(async () => {
@@ -1104,10 +1112,10 @@ describe("Ekklesia shell — mount lifecycle", () => {
       await examplePending;
     });
 
-    // It mounted into its own, already-discarded container — never the live one — and was torn
+    // It mounted into its own, already-discarded isolated root — never the live one — and was torn
     // down immediately, so the newer selection is untouched.
-    expect(liveContainer?.textContent).toBe("calendar mounted");
-    const staleContainer = mountedInto.find((element) => element !== liveContainer);
+    expect(isolatedText(liveContainer)).toBe("calendar mounted");
+    const staleContainer = mountedInto.find((element) => element !== liveMountTarget);
     expect(staleContainer).toBeDefined();
     expect(staleContainer?.isConnected).toBe(false);
     expect(unmountedContainers).toContain(staleContainer);
@@ -1140,7 +1148,7 @@ describe("Ekklesia shell — failure is host-owned", () => {
     await render({ apiBaseUrl, fetch: platformFetch() }, async () => artifact("example", "9.9.9"));
 
     expect(query("module-error")).not.toBeNull();
-    expect(query("module-container")?.textContent).toBe("");
+    expect(isolatedText(query("module-container"))).toBe("");
   });
 
   it("renders its own error when the module's mount throws", async () => {
@@ -1174,7 +1182,7 @@ describe("Ekklesia shell — failure is host-owned", () => {
     });
 
     expect(query("module-error")).toBeNull();
-    expect(query("module-container")?.textContent).toBe("calendar mounted");
+    expect(isolatedText(query("module-container"))).toBe("calendar mounted");
   });
 
   it("survives a module whose cleanup throws, and never re-runs it", async () => {
@@ -1185,6 +1193,7 @@ describe("Ekklesia shell — failure is host-owned", () => {
       }
       return {
         default: {
+          abi: MODULE_UI_ABI_VERSION,
           id: "example",
           mount(element: Element) {
             element.textContent = "example mounted";
@@ -1212,7 +1221,7 @@ describe("Ekklesia shell — failure is host-owned", () => {
     });
 
     expect(cleanupCalls).toBe(1);
-    expect(query("module-container")?.textContent).toBe("calendar mounted");
+    expect(isolatedText(query("module-container"))).toBe("calendar mounted");
 
     // Switch back and away again: the throwing cleanup ran once per mount, never re-invoked for an
     // instance already torn down.
@@ -1223,6 +1232,6 @@ describe("Ekklesia shell — failure is host-owned", () => {
       (query("select-calendar") as HTMLButtonElement).click();
     });
     expect(cleanupCalls).toBe(2);
-    expect(query("module-container")?.textContent).toBe("calendar mounted");
+    expect(isolatedText(query("module-container"))).toBe("calendar mounted");
   });
 });
diff --git a/surfaces/shell/tests/support/isolated-dom.ts b/surfaces/shell/tests/support/isolated-dom.ts
new file mode 100644
index 0000000..9de7a12
--- /dev/null
+++ b/surfaces/shell/tests/support/isolated-dom.ts
@@ -0,0 +1,55 @@
+/**
+ * Test helpers for reaching a mounted module artifact's DOM.
+ *
+ * Since ADR 0040 the generic loader mounts every declared browser artifact inside its own Shadow DOM
+ * root, so a module's elements are deliberately *not* in the host document's tree: `querySelector`
+ * on the shell's container cannot see them, and neither can a stray Dashboard stylesheet. These
+ * helpers walk into those open roots so a shell test can still assert on what a module rendered —
+ * which is exactly what a host's own diagnostics would do, and is why the roots are `open`.
+ *
+ * Deliberately test-only. Production shell code never reaches into an artifact's root; the module
+ * owns its contents.
+ */
+
+const ISOLATION_HOST_SELECTOR = "[data-ekklesia-module-ui]";
+
+function isolatedRoots(root: ParentNode): readonly ShadowRoot[] {
+  return [...root.querySelectorAll(ISOLATION_HOST_SELECTOR)]
+    .map((host) => host.shadowRoot)
+    .filter((shadowRoot): shadowRoot is ShadowRoot => shadowRoot !== null);
+}
+
+/** The first match anywhere in `root`, including inside every mounted artifact's isolated root. */
+export function deepQuery(root: ParentNode, selector: string): Element | null {
+  const direct = root.querySelector(selector);
+  if (direct) {
+    return direct;
+  }
+  for (const shadowRoot of isolatedRoots(root)) {
+    const nested = deepQuery(shadowRoot, selector);
+    if (nested) {
+      return nested;
+    }
+  }
+  return null;
+}
+
+/** Every match anywhere in `root`, host tree first, then each artifact's isolated root in order. */
+export function deepQueryAll(root: ParentNode, selector: string): readonly Element[] {
+  return [
+    ...root.querySelectorAll(selector),
+    ...isolatedRoots(root).flatMap((shadowRoot) => [...deepQueryAll(shadowRoot, selector)]),
+  ];
+}
+
+/** The text an artifact rendered, read from the exact element the loader hands its `mount` — so the
+ * stylesheet the loader installed alongside it is never mistaken for content. */
+export function isolatedText(container: Element | null | undefined): string {
+  return isolatedMountTarget(container)?.textContent ?? "";
+}
+
+/** The element the loader passed to a mounted artifact's `mount`, for identity assertions. */
+export function isolatedMountTarget(container: Element | null | undefined): Element | null {
+  const host = container?.querySelector(ISOLATION_HOST_SELECTOR);
+  return host?.shadowRoot?.querySelector("[data-ekklesia-module-ui-root]") ?? null;
+}
diff --git a/templates/module/package.json b/templates/module/package.json
index 19de833..7cda457 100644
--- a/templates/module/package.json
+++ b/templates/module/package.json
@@ -27,8 +27,10 @@
     "typecheck": "tsc --noEmit --project tsconfig.json && tsc --noEmit --project tsconfig.ui.json && tsc --noEmit --project tsconfig.ui-test.json"
   },
   "dependencies": {
+    "@ekklesia/design-tokens": "workspace:*",
     "@ekklesia/module-sdk": "workspace:*",
     "@ekklesia/module-ui-sdk": "workspace:*",
+    "@ekklesia/ui": "workspace:*",
     "drizzle-orm": "0.45.2",
     "react": "19.2.8",
     "react-dom": "19.2.8"
diff --git a/templates/module/src/display/index.tsx b/templates/module/src/display/index.tsx
index 01f8a9c..81b7e05 100644
--- a/templates/module/src/display/index.tsx
+++ b/templates/module/src/display/index.tsx
@@ -1,27 +1,13 @@
 import { defineModuleUi, type ModuleUiHostContext } from "@ekklesia/module-ui-sdk";
 import { createRoot } from "react-dom/client";
+import displayStyles from "./styles.css";
 
 function ExampleDisplay(props: { readonly context: ModuleUiHostContext }) {
   const mayView = props.context.permissions.has("example.display.view");
 
   return (
-    <main
-      data-testid="example-display"
-      style={{
-        alignItems: "center",
-        background: "#111827",
-        color: "#f9fafb",
-        display: "flex",
-        fontFamily: "system-ui, sans-serif",
-        height: "100%",
-        justifyContent: "center",
-        minHeight: "100vh",
-        padding: "2rem",
-        textAlign: "center",
-        width: "100%",
-      }}
-    >
-      <h1 style={{ margin: 0 }}>
+    <main className="example-display" data-testid="example-display">
+      <h1 className="example-display__headline">
         {mayView
           ? `Example full-screen display (${props.context.moduleId})`
           : "Display unavailable"}
@@ -33,10 +19,15 @@ function ExampleDisplay(props: { readonly context: ModuleUiHostContext }) {
 /**
  * A display uses the same role-neutral ABI as the primary UI. Its separate manifest entrypoint —
  * not a mode flag in `ModuleUiHostContext` — tells a host which artifact it is mounting.
+ *
+ * It declares its own stylesheet, isolated from the primary UI's exactly as it is from the
+ * Dashboard's: two artifacts of the same module share no styles unless they share a source file
+ * (ADR 0040).
  */
 export default defineModuleUi({
   id: "example",
   version: "0.1.0",
+  styles: displayStyles,
   mount(container, context) {
     const root = createRoot(container);
     root.render(<ExampleDisplay context={context} />);
diff --git a/templates/module/src/display/styles.css b/templates/module/src/display/styles.css
new file mode 100644
index 0000000..f65d26c
--- /dev/null
+++ b/templates/module/src/display/styles.css
@@ -0,0 +1,36 @@
+/*
+ * The display artifact's own stylesheet.
+ *
+ * A full-viewport display is a different product experience from the management UI, so it gets its
+ * own rules in its own isolated root — the two artifacts of this one module share no styles unless
+ * they share a source file (ADR 0040).
+ *
+ * `:host { display: block }` is a deliberate override of the platform's transparent default: a
+ * display needs a real box to fill the viewport, and the host installs its base rules before this
+ * stylesheet precisely so an artifact can override them by ordinary cascade order.
+ */
+
+:host {
+  display: block;
+}
+
+.example-display {
+  align-items: center;
+  background: var(--ekklesia-background-page);
+  color: var(--ekklesia-text-primary);
+  display: flex;
+  font-family: var(--ekklesia-typography-family-sans);
+  height: 100%;
+  justify-content: center;
+  min-height: 100vh;
+  padding: var(--ekklesia-spacing-2xl);
+  text-align: center;
+  width: 100%;
+}
+
+.example-display__headline {
+  font-size: var(--ekklesia-typography-size-heading-1);
+  font-weight: var(--ekklesia-typography-weight-bold);
+  line-height: var(--ekklesia-typography-line-height-tight);
+  margin: 0;
+}
diff --git a/templates/module/src/module-styles.d.ts b/templates/module/src/module-styles.d.ts
new file mode 100644
index 0000000..cc9e9ef
--- /dev/null
+++ b/templates/module/src/module-styles.d.ts
@@ -0,0 +1,17 @@
+/**
+ * What `@ekklesia/module-ui-build` turns a local stylesheet import into: that stylesheet's compiled
+ * CSS **text**, as a string (ADR 0040).
+ *
+ * ```ts
+ * import styles from "./styles.css";
+ * export default defineModuleUi({ id, version, styles, mount });
+ * ```
+ *
+ * There is no sidecar file and no URL — the text is compiled into the declared JavaScript artifact,
+ * and the host installs it into the artifact's isolated root at mount. Copy this file into your own
+ * module and include it in your UI TypeScript project.
+ */
+declare module "*.css" {
+  const stylesheet: string;
+  export default stylesheet;
+}
diff --git a/templates/module/src/ui/ExampleApp.tsx b/templates/module/src/ui/ExampleApp.tsx
index 040fe74..c7260c4 100644
--- a/templates/module/src/ui/ExampleApp.tsx
+++ b/templates/module/src/ui/ExampleApp.tsx
@@ -1,9 +1,14 @@
 import type { ModuleUiHostContext } from "@ekklesia/module-ui-sdk";
+import { EmptyState, LoadingState, Panel } from "@ekklesia/ui";
 import { type ReactElement, useEffect, useState } from "react";
 
 /**
  * The template's example screen. It is deliberately tiny, and deliberately host-neutral: it reads
  * everything it needs from the host context and hardcodes no URL, no origin, and no global.
+ *
+ * Presentation comes from the platform's shared elements and this module's own stylesheet
+ * (`./styles.css`, declared through `defineModuleUi({ styles })`), so it matches every other
+ * Ekklesia screen without copying a single colour value (ADR 0039, ADR 0040).
  */
 export function ExampleApp({ context }: { readonly context: ModuleUiHostContext }): ReactElement {
   const [recordCount, setRecordCount] = useState<number | undefined>(undefined);
@@ -39,15 +44,25 @@ export function ExampleApp({ context }: { readonly context: ModuleUiHostContext
   }, [canRead, context.api]);
 
   if (!canRead) {
-    return <p style={{ margin: 0 }}>You do not have access to example records.</p>;
+    return (
+      <EmptyState
+        description="Ask an administrator for the example.records.read permission."
+        title="No access to example records"
+      />
+    );
   }
 
   return (
-    <section style={{ display: "grid", gap: "0.5rem" }}>
-      <h1 style={{ margin: 0 }}>Example module UI</h1>
-      <p style={{ margin: 0 }} data-testid="record-count">
-        {recordCount === undefined ? "Loading records…" : `${recordCount} record(s)`}
-      </p>
-    </section>
+    <Panel description="Everything this module knows about." title="Example module UI">
+      {recordCount === undefined ? (
+        <LoadingState label="Loading records…" />
+      ) : (
+        <div className="example-summary">
+          <p className="example-summary__count" data-testid="record-count">
+            {`${recordCount} record(s)`}
+          </p>
+        </div>
+      )}
+    </Panel>
   );
 }
diff --git a/templates/module/src/ui/index.tsx b/templates/module/src/ui/index.tsx
index 42865fa..2feca3c 100644
--- a/templates/module/src/ui/index.tsx
+++ b/templates/module/src/ui/index.tsx
@@ -1,6 +1,8 @@
 import { defineModuleUi } from "@ekklesia/module-ui-sdk";
+import { EKKLESIA_UI_STYLESHEET } from "@ekklesia/ui";
 import { createRoot } from "react-dom/client";
 import { ExampleApp } from "./ExampleApp.js";
+import ownStyles from "./styles.css";
 
 /**
  * The module's compiled UI entrypoint. `pnpm build` bundles this file, and everything it imports,
@@ -8,10 +10,17 @@ import { ExampleApp } from "./ExampleApp.js";
  *
  * `id` and `version` must match the manifest: a host verifies both before mounting, so a stale
  * artifact is caught rather than rendered.
+ *
+ * `styles` is this artifact's complete stylesheet: the shared elements' rules plus this module's own
+ * (ADR 0040). Both are compiled into the artifact as text and installed by the host into the
+ * artifact's isolated root at mount, so nothing here leaks into the Dashboard and nothing in the
+ * Dashboard reaches in. The design tokens both stylesheets reference are inherited from the host's
+ * root rather than bundled (ADR 0039).
  */
 export default defineModuleUi({
   id: "example",
   version: "0.1.0",
+  styles: `${EKKLESIA_UI_STYLESHEET}\n${ownStyles}`,
   mount(container, context) {
     const root = createRoot(container);
     root.render(<ExampleApp context={context} />);
diff --git a/templates/module/src/ui/styles.css b/templates/module/src/ui/styles.css
new file mode 100644
index 0000000..8358e48
--- /dev/null
+++ b/templates/module/src/ui/styles.css
@@ -0,0 +1,31 @@
+/*
+ * This module's own stylesheet.
+ *
+ * `@ekklesia/module-ui-build` compiles it into the declared browser artifact as text, and the host
+ * installs it into this artifact's isolated root at mount (ADR 0040). There is no sidecar file to
+ * publish and no URL to fetch.
+ *
+ * Two rules make this safe to write freely:
+ *
+ * 1. These selectors apply **only** inside this artifact's own root. `.example-summary` here cannot
+ *    reach the Dashboard, another module, or another contributed renderer — and their rules cannot
+ *    reach in.
+ * 2. Reference Ekklesia's global theme variables; never declare one. A private alias under your own
+ *    prefix, like `--example-*` below, is exactly the supported way to name a local value (ADR 0039).
+ *    The build rejects a stylesheet that declares an `--ekklesia-*` property.
+ */
+
+:host {
+  --example-summary-gap: var(--ekklesia-spacing-sm);
+}
+
+.example-summary {
+  display: grid;
+  gap: var(--example-summary-gap);
+}
+
+.example-summary__count {
+  color: var(--ekklesia-text-secondary);
+  font-variant-numeric: tabular-nums;
+  margin: 0;
+}
diff --git a/templates/module/tests/module-ui.test.ts b/templates/module/tests/module-ui.test.ts
index 5f734f3..bed19d9 100644
--- a/templates/module/tests/module-ui.test.ts
+++ b/templates/module/tests/module-ui.test.ts
@@ -207,6 +207,26 @@ describe("module UI build pipeline", () => {
     await (unmount as () => Promise<void>)();
   });
 
+  it("declares its compiled stylesheet on the artifact rather than beside it", async () => {
+    const namespace = (await import(artifactHref)) as unknown;
+    const definition = parseModuleUiExport(namespace, manifestIdentity);
+
+    // The module's own CSS *and* the shared elements' rules travel inside the one declared
+    // JavaScript artifact, as text (ADR 0040). A host installs them; nothing fetches a second file.
+    expect(definition.styles).toContain(".example-summary");
+    expect(definition.styles).toContain(".ek-panel");
+    expect(definition.styles).toContain("var(--ekklesia-text-secondary)");
+    // A module references the global theme and never declares part of it (ADR 0039).
+    expect(definition.styles).not.toMatch(/--ekklesia-[a-z0-9-]+\s*:/u);
+    await expect(readFile(path.join(path.dirname(artifactPath), "index.css"))).rejects.toThrow();
+
+    const displayNamespace = (await import(displayArtifactHref)) as unknown;
+    const displayDefinition = parseModuleUiExport(displayNamespace, manifestIdentity);
+    // Two artifacts of one module: separate roots, separate stylesheets.
+    expect(displayDefinition.styles).toContain(".example-display");
+    expect(displayDefinition.styles).not.toContain(".example-summary");
+  });
+
   it("renders a denied state from the host's presentation-only permission hint", async () => {
     const namespace = (await import(artifactHref)) as unknown;
     const definition = parseModuleUiExport(namespace, manifestIdentity);
@@ -219,7 +239,7 @@ describe("module UI build pipeline", () => {
     );
     await flush();
 
-    expect(container.textContent).toContain("do not have access");
+    expect(container.textContent).toContain("No access to example records");
     // A hidden control is a usability choice, not enforcement: the module simply did not ask.
     // Server-side authorization is what actually protects the route.
     expect(requested).toEqual([]);
diff --git a/templates/module/tsconfig.ui.json b/templates/module/tsconfig.ui.json
index d5bb7eb..d9861d8 100644
--- a/templates/module/tsconfig.ui.json
+++ b/templates/module/tsconfig.ui.json
@@ -4,5 +4,11 @@
     "noEmit": true,
     "types": []
   },
-  "include": ["src/ui/**/*.tsx", "src/ui/**/*.ts", "src/display/**/*.tsx", "src/display/**/*.ts"]
+  "include": [
+    "src/module-styles.d.ts",
+    "src/ui/**/*.tsx",
+    "src/ui/**/*.ts",
+    "src/display/**/*.tsx",
+    "src/display/**/*.ts"
+  ]
 }
diff --git a/tests/boundaries.test.ts b/tests/boundaries.test.ts
index 49b9424..a762e94 100644
--- a/tests/boundaries.test.ts
+++ b/tests/boundaries.test.ts
@@ -61,7 +61,7 @@ describe("repository dependency boundaries", () => {
     expect(result.violations.map((violation) => violation.rule)).toContain(expectedRule);
   });
 
-  it("rejects stylesheet and case-insensitive view files inside a module, but not TSX", async () => {
+  it("rejects a misplaced stylesheet and case-insensitive view files inside a module, but not TSX", async () => {
     const result = await checkBoundaries(path.join(fixtures, "module-has-ui"));
     const uiViolations = result.violations.filter(
       (violation) => violation.rule === "modules-do-not-own-ui",
@@ -70,10 +70,17 @@ describe("repository dependency boundaries", () => {
     expect(uiViolations).toHaveLength(2);
     expect(uiViolations.map((violation) => violation.from)).toEqual(
       expect.arrayContaining([
+        // A stylesheet outside a compiled browser source root is a file no build reads and no host
+        // loads — the undeclared sidecar ADR 0023 rejected, just relocated (ADR 0040).
         "modules/calendar/src/calendar.css",
         "modules/calendar/src/view.ASTRO",
       ]),
     );
+    // Inside `src/ui/`, a plain stylesheet is legitimate source the canonical builder compiles into
+    // the declared artifact.
+    expect(uiViolations.map((violation) => violation.from)).not.toContain(
+      "modules/calendar/src/ui/styles.css",
+    );
     // React is the supported module UI authoring model (ADR 0022), so a module's own `.tsx` is
     // legitimate source and must no longer be reported.
     expect(uiViolations.map((violation) => violation.from)).not.toContain(
diff --git a/tests/fixtures/boundaries/module-has-ui/modules/calendar/src/ui/styles.css b/tests/fixtures/boundaries/module-has-ui/modules/calendar/src/ui/styles.css
new file mode 100644
index 0000000..8a751a6
--- /dev/null
+++ b/tests/fixtures/boundaries/module-has-ui/modules/calendar/src/ui/styles.css
@@ -0,0 +1,4 @@
+/* Legal since ADR 0040: a plain stylesheet inside a compiled browser source root. */
+.calendar-day {
+  color: var(--ekklesia-text-primary);
+}