Currently, custom elements lack the ability to set default accessibility semantics that built-in elements have. This feature allows for the setting of defaults through the custom element's ElementInternals via ARIA reflection, without affecting elements that have accessibility attributes set on them directly.
Bug: Bug 1665151
Standards Body: WHATWG
Standards position: https://github.com/mozilla/standards-positions/issues/201
Platform coverage: All
Preference: This feature relies on Aria Reflection and so is currently gated behind the accessibility.ARIAReflection.enabled pref.
DevTools bug: n/a
web-platform-tests: A surface-level-only test was provided at https://github.com/web-platform-tests/wpt/pull/18223. Deeper tests do not appear possible with WPT currently: https://github.com/web-platform-tests/wpt/issues/7753, so Gecko-specific tests are being written as well.