Add platform-provided behaviors for custom elements & submit behavior

[anaskim]

#12150

This change introduces platform-provided behaviors, a mechanism that allows autonomous custom elements to adopt built-in HTML element capabilities via attachInternals(). The initial element behavior is HTMLSubmitButtonBehavior, which gives a custom element submit button semantics including form submission, activation, focusability, and an implicit ARIA role.

Explainer: https://github.com/MicrosoftEdge/MSEdgeExplainers/blob/main/PlatformProvidedBehaviors/explainer.md

• At least two implementers are interested (and none opposed):
  • Chromium: Chrome Status entry & implementation bug
  • Webkit standards position
  • Mozilla standards position
• Tests are written and can be reviewed and commented upon at:
  • https://github.com/web-platform-tests/wpt/blob/master/custom-elements/element-internals-behaviors.tentative.html
  • https://github.com/web-platform-tests/wpt/blob/master/custom-elements/form-associated/ElementInternals-submit-behavior-dialog.tentative.html
  • https://github.com/web-platform-tests/wpt/blob/master/custom-elements/form-associated/ElementInternals-submit-behavior.tentative.html
  • TODO: PR to make them non-tentative.
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/486928684
  • Gecko: …
  • WebKit: …
• 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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/custom-elements.html ( diff )
/dom.html ( diff )
/form-control-infrastructure.html ( diff )
/forms.html ( diff )
/index.html ( diff )
/interaction.html ( diff )
/semantics-other.html ( diff )