Docs: reconcile docs/css-support.md with the 2026-09-01 compatibility issue audit

[mplsllc]

Status

The issue tracker has been reconciled against the current workflow tree, but docs/css-support.md still contains a few stale property-level statements from earlier rounds. The issue-level source of truth is now #267; the support matrix should be brought into exact agreement with it and with hardware-verified issue closures.

Known examples to verify/correct:

• image-rendering / box-decoration-break still appear stale relative to completed hardware-verified CSS: image-rendering + box-decoration-break #256.
• transitions must describe Round 2A complete + opacity temporal presentation hardware-verified, not the old parse/drop state.
• partial @font-face, background SVG sizing, shorthand background-origin, hyphens:auto, and newly tracked conic-gradient() must point to their current owners.
• selector rows should distinguish stylesheet :is/:where/:has (:is() / :where() / :has() real matching (V2) #163), :focus-within (CSS: Implement :focus-within pseudo-class selector #324), and :focus-visible (CSS: implement :focus-visible pseudo-class selector #329).

Scope

Audit every non-✅ row and every issue reference in docs/css-support.md against current source and the open/closed issue state. Do not change a status from partial/unsupported to full without source + hardware evidence.

Acceptance

• No support-matrix row points to a closed issue as if it were still the active gap owner unless the row explicitly describes a completed subpart.
• Every partial/unsupported CSS row has an open issue owner or an explicit accepted/not-planned rationale.
• META: current CSS + JavaScript/DOM compatibility gap index #267 and docs/css-support.md tell the same story at different levels of detail.
• Documentation-only commit pushed on workflow.