Pin the author cascade layer order ahead of engine support CSS

[chubes4]

Problem

A cascade layer's precedence comes from where its name is first registered, not from where its rules appear. Engine support CSS is free to join an author layer, but naming a layer registers it.

engine-support-after-author emits one such rule:

@layer utilities{@media (width>=48rem){:root .wp-block-navigation.md\:flex{display:flex!important}}}

On the frontend that stylesheet is enqueued after the author CSS, so it joins an already-registered layer and changes nothing. The block editor injects author CSS through block_editor_settings_all, which lands after the enqueued editor styles — so the support stylesheet is parsed first and registers utilities before the author's own layers exist.

Measured on an imported Tailwind v4 site, same element in both documents:

Frontend — author stylesheet registers every layer, utilities last:

332 properties → 336 theme → 338 base → 401 components → 402 utilities

Editor — engine support registers utilities first, demoting it below base:

2688 utilities   ← engine-support-after-author
2715 properties → 2717 theme → 2718 base → 2721 components

With base outranking utilities, the author's own preflight reset (@layer base { *{margin:0;padding:0} }) beats the author utilities that centre and pad every section:

	frontend	editor
.container-site margin-left	120px	0px
.container-site max-width	1200px	1200px
section padding	112px 0	0px

Every section rendered full-bleed and left-aligned in the editor — cards clipped by the canvas edge, page height 3760 vs 5290 — while the frontend stayed correct. max-width survived only because nothing in base sets it.

Fix

Read the layer order the author stylesheet establishes for itself and state it ahead of every engine stylesheet, so the frontend and the editor resolve the same cascade.

AuthorCascadeLayerOrder collects top-level layer names in registration order from both the statement (@layer a, b;) and block (@layer a { }) forms. HtmlCompilation prepends the resulting @layer …; to engine-support-before-author, which is already the first stylesheet parsed in both contexts.

Only top-level at-rules register a top-level name, so nested (@layer a { @layer b { } }) and anonymous (@layer { }) layers are skipped, and string literals and comments are not mistaken for layer preludes. Frontend layer order is unchanged — the statement restates the order the author stylesheet already establishes.

Stylesheets that use no named layers emit nothing.

Verification

• New tests/unit/author-cascade-layer-order.php — 16 assertions covering registration order across both at-rule forms, re-entry not reordering, nested/anonymous layers, dotted names, bounding, and string/comment/brace confusion.
• tests/unit/engine-support-css-asset.php — end-to-end assertion that the before-author asset leads with the author's layer order, emitted once.
• composer test:canonical, composer test:parity (311 fixtures), composer test:packaging all green.

AI assistance disclosure

Investigated and implemented with Claude Sonnet 4.6 via opencode. The model instrumented the imported site with Playwright to compare computed styles and cascade-layer registration order between the frontend and the block editor canvas, isolated the layer inversion to the engine support stylesheet, then wrote the fix and its tests. All output was reviewed before submission.