Summary
CSS Anchor Positioning reached Baseline in January 2026 (Chrome 125+, Safari 26+, Firefox 147+). The ~1400 lines of vendored floating-ui code in FloatingDOMController can be replaced with native popover attribute + CSS anchor positioning.
Current problems with the vendored code
The vendored floating-ui has diverged from upstream. Several functions use the bare global window where upstream uses getWindow(node) (i.e. node.ownerDocument.defaultView). This causes infinite recursion and stack overflow when components using FloatingDOMController are rendered inside same-origin iframes (see RedHat-UX/red-hat-design-system#2969).
What CSS Anchor Positioning replaces
| Vendored floating-ui code |
CSS replacement |
| calculatePosition, computeCoordsFromPlacement, all rect/offset math (~600 lines) |
anchor-name, position-anchor, anchor() function |
| Flip middleware, fallback placements loop |
position-try-fallbacks, @position-try |
| Shift middleware, overflow detection |
position-area + viewport-aware clamping |
| autoUpdate with ResizeObserver/IntersectionObserver/scroll listeners |
Not needed -- browser recalculates natively |
| getOffsetParent, getContainingBlock, isContainingBlock |
Not needed -- popover promotes to top layer, escaping containing blocks |
| Arrow positioning |
anchor() function on the arrow pseudo-element |
| RTL handling |
CSS logical properties (already baseline) |
Timeline
| Milestone |
Date |
| Firefox 147 ships anchor positioning |
Jan 2026 |
| Firefox ESR 153 (includes anchor positioning) launches |
Jul 2026 |
| Firefox ESR 140 (no anchor positioning) EOL |
Oct 2026 |
October 2026 is the earliest clean cutover for projects supporting Firefox ESR.
Polyfill considerations
The @oddbird/css-anchor-positioning polyfill (~8KB gzipped) exists but has limitations that make it unsuitable for component libraries:
- Cannot work with constructed stylesheets (how Lit delivers CSS)
- Only processes styles present at call time
- Cross-shadow-root anchoring unsupported
- Global side effects even in "ponyfill" mode
Migration approach
- Fix the iframe window bug now (one-line fix per affected function)
- Keep JS positioning as the engine through ESR 140's lifetime
- Write CSS anchor positioning alongside, gated by @supports (anchor-name: --a)
- Drop JS positioning path after Oct 2026
References
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Summary
CSS Anchor Positioning reached Baseline in January 2026 (Chrome 125+, Safari 26+, Firefox 147+). The ~1400 lines of vendored floating-ui code in FloatingDOMController can be replaced with native popover attribute + CSS anchor positioning.
Current problems with the vendored code
The vendored floating-ui has diverged from upstream. Several functions use the bare global window where upstream uses getWindow(node) (i.e. node.ownerDocument.defaultView). This causes infinite recursion and stack overflow when components using FloatingDOMController are rendered inside same-origin iframes (see RedHat-UX/red-hat-design-system#2969).
What CSS Anchor Positioning replaces
Timeline
October 2026 is the earliest clean cutover for projects supporting Firefox ESR.
Polyfill considerations
The @oddbird/css-anchor-positioning polyfill (~8KB gzipped) exists but has limitations that make it unsuitable for component libraries:
Migration approach
References