Make topmargin and leftmargin attributes apply to both sides

[zcorpan]

Fixes #11879.

• At least two implementers are interested (and none opposed):
  • Gecko
  • Already implemented in WebKit, Chromium
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium: N/A
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1997310
  • WebKit: N/A
• MDN issue is filed: … (https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/body#deprecated_attributes could be updated)
• The top of this comment includes a clear commit message to use.

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

---

/rendering.html ( diff )

[zcorpan]

Maybe we should fix #2486 (comment) at the same time?

[annevk]

Are there tests for the rightmargin and bottommargin attributes?

Also, I think the table is a bit confusing now as the heading says "Property". Making it "Properties" would help, but perhaps it would also require tweaking the introduction of the table.

[annevk]

I don't think we should couple this change with the privacy change by the way. Let's first align and then contemplate a change in how this works separately.

[zcorpan]

Existing tests: https://wpt.fyi/results/html/rendering/non-replaced-elements/the-page?label=experimental&label=master&aligned

The priority order was changed in #2489 but per https://issues.chromium.org/u/1/issues/41310601#comment11 it seems @josepharhar wanted to wait with making implementation changes until the cross-origin communication channel fix is done. But I can open a separate PR for that.