# 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*.