Wrap "activate view transition" with script prepare/clean up

[noamr]

This ensures that we perform a microtask checkpoint (and set the incumbent script) right after resolving/rejecting the ViewTransition promises.

It's necessary as those promises need to be resolved before the other rendering operations, mainly style/layout.

• At least two implementers are interested (and none opposed):
  • Chromium
  • Gecko (See View Transitions API mozilla/standards-positions#677. This is a spec bug fix rather than a new feature).
• Tests are written and can be reviewed and commented upon at:
  • https://wpt.fyi/results/css/css-view-transitions/navigation/pagereveal-microtask-sequence.html?label=experimental&label=master&aligned
• Implementation bugs are filed:
  • Chromium: already implemented
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=css-view-transitions-2
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=275486
• MDN issue is filed: N/A, https://developer.mozilla.org/en-US/docs/Web/API/Window/pagereveal_event sufficiently covers this.
• 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 )

[domenic]

Editorial nits, content LGTM. Please also fill out the issue template appropriately.

[khushalsagar]

Thanks. LGTM!

[noamr]

@domenic I applied the nit suggestions and also filled in the boxes. Ready to merge?