Add pagereveal event

[noamr]

Add pagereveal event, and the PageRevealEvent interface.
The pagereveal event is fired at the beginning of the first rendering opportunity after activation (initial load or reactivation).
It is a way for the author to execute some JS that affects the presentation "just in time" for the first frame.
If there is an inbound cross-document view transition, the reveal event holds a reference to the ViewTransition object.

Closes #9315.

• At least two implementers are interested (and none opposed):
  • Chromium
  • Gecko - see comments about the RFP being enough for cross-document view transitions (which include PageReveal already in the explainer), and comments on Proposal: fire an event before the first rendering opportunity after activation #9315.
• Tests are written and can be reviewed and commented upon at:
  • view-transition: Move pagereveal tests to WPT web-platform-tests/wpt#42669
• Implementation bugs are filed:
  • Chromium: https://bugs.chromium.org/p/chromium/issues/detail?id=1466250
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1860854
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=263602
• MDN issue is filed: Add documentation for pagereveal event mdn/mdn#467
• The top of this comment includes a clear commit message to use.

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

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/browsing-the-web.html ( diff )
/index.html ( diff )
/indices.html ( diff )
/infrastructure.html ( diff )
/nav-history-apis.html ( diff )
/webappapis.html ( diff )

[bokand]

lgtm!

[noamr]

@smaug---- @domenic I think this is ready for review, I checked the appropriate boxes.

[noamr]

@domenic I think this is ready for another run.

[domenic]

Sorry for the delay. Found a few new nits, and several old ones did not get resolved, including:

• Add pagereveal event #9818 (comment)
• Add pagereveal event #9818 (comment)

[noamr]

Sorry for the missed comments, apparently some comments appear in the review pane and not in the comments pane...

[domenic]

LGTM. I did push one more commit to move the event interface back into event interfaces.

Since Gecko has been quite involved in reviewing this, and hasn't yet given a clear "LGTM", I'll give folks like @smaug---- and @zcorpan another day or two to weigh in before we merge. But I agree with your current assessment that there seems to be Gecko support, so unless I hear otherwise I'll default to merging soon.