Clarify that <details> toggle events happen during parsing

[domenic]

Closes #4500.

• At least two implementers are interested (and none opposed):
  • Already implemented everywhere
• Tests are written and can be reviewed and commented upon at:
  • HTML: Test that <details open> fires the event even from the parser. web-platform-tests/wpt#16244 for open attribute
  • https://github.com/web-platform-tests/wpt/blob/master/html/semantics/interactive-elements/the-details-element/name-attribute.html for name groups
• Implementation bugs are filed: Not needed
• MDN issue is filed: Not needed
• The top of this comment includes a clear commit message to use.

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

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/interactive-elements.html ( diff )

[annevk]

Adding do not merge yet as I cannot reproduce this in XMLHttpRequest.

[domenic]

I've rebased this and web-platform-tests/wpt#16244 and they should be ready to merge.

Adding do not merge yet as I cannot reproduce this in XMLHttpRequest.

XMLHttpRequest documents are parsed before anyone has a chance to attach event listeners to them, and do not run inline <script>s. So I do not think this is relevant.