WebKit implements the following non-standard APIs:
- the `search` event
Neither Firefox nor Edge implement these.
Blink *does* implement these APIs, but the Blink maintainers are currently
investigating the impact of its removal . That issue includes references
to the `incremental` attribute, but I have not seen any indication of its
existence in WebKit.
WebKit currently has at least one open bug concerning the behavior of this
Tests added in https://github.com/web-platform-tests/wpt/pull/19889
The test results suggest WebKit does support an incremental attribute.