Change focus delegate for dialog to use flat tree to find focusable descendant

[dizhang168]

• At least two implementers are interested (and none opposed):
  • Chromium (Focus delegate algorithm doesn't consider elements assigned to slot #9245 (comment))
  • Gecko
  • Webkit
• Tests are written and can be reviewed and commented upon at:
  • WPT test PR: New dialog focus steps using Flat Tree web-platform-tests/wpt#51033
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/40369887
  • Gecko: TBA
  • WebKit: TBA
• Corresponding HTML AAM & ARIA in HTML issues & PRs:
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

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

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/interaction.html ( diff )