Complete dynamic CSSOM for nested, imported, and constructed stylesheets

[wieslawsoltes]

Current dynamic CSSOM checkpoint — 20 September 2026

Exact merged heads: WebScene a46b62e14eb197603f7e227c1558afb9088633b0; AppScene 97702daeb9ad71341ced7226f0aa966dc77ea53c; unchanged Code OSS 645f29cc3176500b4b5762ba887cf2a7f0ffdf2c; local vscode-demo e4b98e5470cd7856aff3dfd6843c8e125d7db933 (clean, intentionally unpushed). Open consolidations remain WebScene #76 085bba760fa571c8f08ea866dc5dd1ec27c4d376, AppScene #65 ce66ef6ceb776b3f14e129f853bfc08b5c5e608c, and vscode-demo #1; do not merge them. One-agent mode is active.

All focused dynamic-CSSOM children remain closed with implementations merged: grouping/media/selector replacement, constructed and adopted sheets, imports, namespaces, containers, keyframes and font-face behavior. No additional source slice is currently known under this owner.

Remaining work is exact-package execution of the authored browser/native mutation, ordering, lifecycle, memory and unchanged Code OSS acceptance gates. Open a focused child only when current evidence proves another CSSOM defect.

Current implementation status — 19 September 2026

Focused CSSOM work is merged through namespace-rule reflection and selector namespace stages #518/#519: type/universal matching landed in #521 (276d5b77), and namespace-qualified attribute matching landed in #525 (346f1412) with expanded DOM attribute identity, cache schema, invalidation, and authored WPT/native sources.

PR #528 merged the final planned namespace-context slice for supported functional pseudo-class selector lists as 9f2d4830, closing #520. The epic remains open for the deferred cumulative Chromium/native mutation, invalidation, performance, memory, teardown, exact-package, and CI gates. Unchanged Code OSS contains no @namespace usage at the pinned revision, so these are reusable standards fixes rather than product patches.

Parent tracker: #235

Problem

WebScene supports important dynamic style paths, but nested rule mutation, imported sheets, constructed/adopted sheets, owner detach/reattach, and complete CSSOM reflection are not yet qualified as one coherent feature surface.

Proposed work

• Inventory CSSStyleSheet, CSSRuleList, grouping rules, declaration mutation, insertRule, deleteRule, replace, and replaceSync behavior.
• Implement product-neutral nested/grouping, imported, and constructed/adopted stylesheet ownership and mutation.
• Preserve rule identity/order and correctly invalidate affected documents/subtrees.
• Define navigation/realm teardown and detached-owner lifetimes.
• Add Chromium/native contracts and mutation performance gates.

Acceptance

• CSSOM API shape, exceptions, rule order, and serialization match the covered browser contract.
• Insert/delete/replace/declaration mutations update computed style and rendering immediately.
• Imported and adopted sheets have proven ownership and teardown behavior.
• Detached/reattached owners cannot leave stale cache entries.
• Work is measured against the post-Fix catalog script and logical sizing compatibility #148 invalidation baseline and linked from [Epic] CSS features, compatibility, validation, and performance #235.

[wieslawsoltes]

Started the first bounded missing semantic as native sub-issue #330: live CSSGroupingRule mutation for nested @media rules.

Current top-level owner-backed CSSOM already covers live cssRules, insert/delete, declaration mutation, enable/media toggles, detach/reattach, synchronous observability, batched publication, and queue/memory bounds. The first unsupported operation on that existing surface is nested grouping inspection/mutation. Chromium control evidence and the exact product-neutral scope, non-overlap boundary, intended files, and acceptance gates are recorded in #330.

Deferred to later #239 children: @import/@namespace, constructed/adopted sheets, replace/replaceSync, other grouping-rule types, and cumulative cross-realm ownership.

[wieslawsoltes]

Selected the next bounded owner-backed CSSOM operation as #359: live CSSStyleRule.selectorText mutation. Chromium 153 proves synchronous top-level/nested recascade and serialization, invalid assignment as a no-op, stable DOM text, and detached-rule mutation without recascade. The four planned paths are isolated from active #312/#248 and open PRs #354/#357/#76; constructed/adopted multi-root ownership remains a later child.

[wieslawsoltes]

Focused child #359 merged through #361 at 92799fb39b1d94d901a6fbe81ee5023eb9f2ba9a. Owner-backed top-level and nested CSSStyleRule.selectorText now validate with the native selector parser, recascade synchronously, preserve declaration mutation and rule identity, ignore invalid assignments, and keep detached mutations isolated. Chromium/native 6/6 plus bounded perf/heap/RSS/realm lifecycle gates passed. #239 remains open for constructed/adopted sheets, imports, replace APIs, and remaining CSSOM reflection.

[wieslawsoltes]

Opened focused child #362 for live CSSMediaRule.media / MediaList mutation. The current owner-backed implementation already has condition state, nested serialization, synchronous staged publication, detach handling, and realm-local interfaces; the standard SameObject media reflection is the next isolated missing semantic. Planned paths are limited to the CSSOM compatibility header, its native browser test, and new dedicated contract/profile files, with zero overlap with #312 PR #354, #248 File System Access work, or #76.

Constructed/adopted sheets remain deferred because current native stylesheet identity and staged publication are keyed to connected style/link owner nodes; correct multi-document/shadow-root adoption needs a new native ownership primitive rather than an owner-backed adapter shortcut.

[wieslawsoltes]

Focused child #362 merged through PR #365 at c4c4ac915918d44919d573539e89e7ace551aa5c. Owner-backed top-level and nested media rules now expose a stable realm-local MediaList with PutForwards assignment, normalized list reflection, append/delete mutation, synchronized serialization, synchronous recascade, and detached-rule isolation. Chromium/native passed 6/6; the exact-head 20-cycle p95/max was 0.209/0.342 ms, and the 100-cycle heap/RSS/rule/queue lifecycle gate passed. Linux, macOS, portable V8, and native document runners were green.

#239 remains open for constructed/adopted sheet ownership, replace/replaceSync, imports/namespaces, and remaining CSSOM reflection. Constructed adoption still depends on a native owner-independent stylesheet identity because current native wrappers/publication are keyed to connected style/link nodes.

[wieslawsoltes]

Opened native child #366 for the next bounded #239 semantic: exact replaceSync() / replace() rejection behavior on existing owner-backed stylesheets.

Chrome 153 and WPT CSSStyleSheet-constructable-replace-on-regular-sheet.html require a regular <style>/linked sheet to reject replacement (replaceSync throws NotAllowedError; replace returns a rejected realm-local Promise), including after owner detach, without changing rules, computed style, owner text, or invalidation/publication state. Web IDL missing-argument and Symbol conversion behavior is included.

The implementation is limited to the CSSOM compatibility header, its native browser test, and a new dedicated contract/profile. It is disjoint from open docs PR #76, #248 File System Access paths, #266 resources, AppScene, and vscode-demo.

Successful replacement and constructed/adopted sheets remain blocked on an owner-independent native stylesheet identity plus per-document/shadow-root adoption registries. Current wrappers and staged publication are keyed to connected style/link owner nodes, so #366 will not claim constructed-sheet support or mutate a regular sheet.

[wieslawsoltes]

Implementation is open as PR #369 at exact base d521dd05f217eb1a43604b357346197d60b5b294, head bce745d100f4e4b950cc20b12fcd3b0a1bfc5106.

The focused four-path change adds exact constructed-only guards for existing owner-backed sheets: replaceSync throws synchronously, replace returns a rejected Promise, missing/Symbol arguments follow Web IDL, and attached/detached main/iframe rules remain unchanged. It does not expose a constructed-sheet constructor or claim adoption support.

Current evidence:

• Chrome 153 direct contract: 6/6;
• native WebPlatformSubset contract: 6/6;
• direct native correctness filter: pass;
• 64-rule / 20-cycle denial p95/max: 0.023/0.295 ms, with exact zero layout, publication, scene-build, native-rule, and pending-scene deltas;
• 100-cycle main/iframe lifecycle: 200 async rejections, flat V8 heap, RSS +17.74 MiB, drained native rules/wrappers.

The branch was rebased after #367; its four paths remain disjoint from #248, #266, open docs PR #76, AppScene, and vscode-demo.

[wieslawsoltes]

PR #369 is rebased after disjoint #368 at exact base 83b2c4e8b3c1cc71e4f4a8f67b8db470a9031f15, head 24dd08dd6e14607a17ffbf80aa64834f947307b5. Local direct correctness/performance and native 6/6 were rerun unchanged at this head; the PR body carries the updated evidence.

Merge is held for runner coordination. All three exact-head Linux workflows hit the same self-hosted-host exhaustion outside this change: CI fails setup-dotnet extraction, portable V8 completes adapter/cold-process contracts then cannot write/link CMake temp files, and native document configures Clang then cannot write Rust temp artifacts. Every failure reports No space left on device; exact predecessor #368 passed Linux/macOS CI minutes earlier. Exact-head macOS remains queued/running. No code workaround or scope expansion is planned.