Don't queue a microtask from parallel in update the image data

[zcorpan]

Also modernize how algorithm flags are written.

Fixes #11812.

• At least two implementers are interested (and none opposed):
  • This is a bug fix.
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
• The top of this comment includes a clear commit message to use.

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

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/images.html ( diff )

[domfarolino]

This looks good, thanks. I like the introduction of the "internal" steps.

[domfarolino]

It might be good to hold off landing this until we feel more comfortable with #11812 (comment), i.e., discovering whether or not implementations queue tasks or microtasks after a document becomes fully active, to continue to the image updating algorithm, and figure out if that's observable or not (I suspect it is).