An authored matrix of overlay capability gates, evidence, and ordinary alternatives.
Reviewed . Support is defined by web-platform capabilities, not a consuming application or one browser brand.
These are observed property/syntax checks from locked test browsers, not proof of every interaction. The same semantic fixtures test native behavior across all three engines.
| Feature and authoritative source | Package policy | chromium 151.0.7922.34 | firefox 153.0 | webkit 26.5 | Fallback |
|---|---|---|---|---|---|
| Native dialog | Implemented — required native capability | Detected | Detected | Detected | Use the complete ordinary destination when unavailable. |
| commandfor / command | Implemented — required native capability | Detected | Detected | Detected | Use the complete ordinary destination when unavailable. |
| closedby | Implemented — required native capability | Detected | Detected | Detected | Use the complete ordinary destination when unavailable. |
| Popover | Implemented — required native capability | Detected | Detected | Detected | Use the complete ordinary destination when unavailable. |
| popovertarget | Implemented — required native capability | Detected | Detected | Detected | Use the complete ordinary destination when unavailable. |
| CSS anchors | Capability-gated CSS enhancement | Detected | Detected | Detected | Retain bounded placement or snap presentation; supplemental RTL retains normal flow. |
| Position tries | Capability-gated CSS enhancement | Detected | Detected | Detected | Retain bounded placement or snap presentation; supplemental RTL retains normal flow. |
| Discrete transitions | Capability-gated CSS enhancement | Detected | Detected | Detected | Retain bounded placement or snap presentation; supplemental RTL retains normal flow. |
| Interest invokers | Deliberately excluded | Detected | Not detected | Not detected | Not emitted, even where detected. Supplemental CSS and ordinary links are the fallback. |
Choose and verify your browser support
- anchoredPopoverPresentation: anchorPositioning, positionFallbacks
- dialogFamily: dialog, dialogInvokerCommands, dialogClosedBy
- overlayMotion: discreteTransitions
- popoverFamily: popover, popoverTarget
Require the complete native capability set for the component you choose. Test your own browsers with keyboard, pointer and assistive technology, and always provide an ordinary destination. Do not infer behavior from a user-agent string.
Update evidence independently
Review authoritative sources when browser locks or fallback policy change. Update the authored package manifest, run the same cross-engine semantic suites, then regenerate and check the demo-only record with node scripts/record-overlay-capabilities.mjs. No current-visitor sniffing, component shim or event runtime is installed.
Scoped RTL supplemental anchors use normal-flow fallback across engines. Native keyboard preferences may skip links, and native focus restoration can vary. These limits are tested and documented rather than repaired with scripts.
Support and exact fallback
| Feature | Locked-engine evidence | When missing |
|---|---|---|
| Semantic page baseline | Controller and static-export tests exercise the same closed route. | Without optional gallery presentation, complete content and ordinary destinations remain in document order. |
Without optional gallery presentation, complete content and ordinary destinations remain in document order.