Add focusin and focusout events

[josepharhar]

These events are already implemented in browsers and are documented on MDN. This PR also adds onfocusin and onfocusout event handler content attributes.

https://github.com/whatwg/html/issues/3514 tracks this and other focus events.

Fixes https://github.com/whatwg/html/issues/10234

• [x] At least two implementers are interested (and none opposed):
  • Chromium already implements
  • WebKit already implements
  • Gecko already implements
• [x] Tests are written and can be reviewed and commented upon at:
  • https://wpt.fyi/results/uievents/order-of-events/focus-events
  • https://github.com/web-platform-tests/wpt/pull/45432
• [x] Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/330759712
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1886428
  • WebKit: Already implements onfocusin/onfocusout
• [x] MDN issue is filed: https://github.com/mdn/content/issues/32884
• [x] The top of this comment includes a clear commit message to use.

(See WHATWG Working Mode: Changes for more details.)

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/dom.html ( diff ) /indices.html ( diff ) /interaction.html ( diff ) /webappapis.html ( diff )

[domenic]

Browsing Blink source code I found these interesting if statements:

https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/page/focus_controller.cc;l=434;drc=2a6270727b6a7bbcd17ab585dc81ea4fc5a44d71

https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/page/focus_controller.cc;l=445?q=DOMFocusIn&ss=chromium

https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/core/dom/document.cc;l=5601-5607;drc=ebef5d0e968e4b5519062305af9856f53c41eb2f

I think these can have interesting impacts if you call focus()/blur() inside of a focus/blur event handler. Doing so might prevent focusin / focusout from being fired.

Should we add that to the spec? Do we have tests for such cases?

[josepharhar]

Should we add that to the spec?

Nice, definitely!

Do we have tests for such cases?

I removed the logic in a chromium patch and am running all tests on it now to find out