add(foundations): CSS anchor positioning page#54
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New spec page documenting CSS anchor positioning (anchor-name / position-anchor / position-area, CSS Anchor Positioning Module Level 1). Baseline newly available since early 2026; pairs with the Popover API for tethering tooltips, menus, and popovers to their trigger with no JS. - Adds src/content/spec/foundations/anchor-positioning.md (status: recommended) - Wires anchor-positioning into popover-api relatedSlugs - Changelog "added" entry, OG images regenerated Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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What
New spec page: CSS anchor positioning under
foundations.Documents the
anchor-name/position-anchor/position-areamechanism — tethering tooltips, menus, and popovers to their trigger element with pure CSS, acrossoverflow/z-index/transformboundaries, with@position-tryviewport-aware fallbacks. Replaces JS positioning libraries (Popper / Floating UI).Why now
CSS anchor positioning reached Baseline "newly available" in January 2026 (Chrome, Safari 18.2+, Firefox) — the freshest web-platform feature in today's scan. We already reference it in passing on the Popover API page but had no standalone topic. It's the natural positioning companion to our existing popover-api / view-transitions / scroll-driven-animations "modern platform feature" pages.
Primary source
/Guides/Anchor_positioning/UsingURL +anchor-name) and Chrome for Developers for context.Status
recommended— a progressive enhancement (gated behind@supports, with a static fallback), not a platform requirement. The page is explicit that you keep a usable fallback until support is universal, per the status discipline ("requiredonly if the platform breaks without it").Changes
src/content/spec/foundations/anchor-positioning.md(new, ~480 words, canonical sections, British English)relatedSlugswired reciprocally intopopover-apiaddedentry (2026-06-24-anchor-positioning.md)og-default/checklist/spec/foundationscategory)Verification
npm run buildpasses (149 pages indexed).npm run lint0 errors (1 pre-existing warning, unrelated file).npm run format:checkclean.Opened as a draft for maintainer review — not for auto-merge.
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