Remove "compatibility caseless" matching

[domenic]

This fixes #1666. As discussed there, browsers are not interoperable
about what type of Unicode case-insensitivity they implement here, with
WebKit even using case-sensitive matching for the radio button case (but
not for the image map case). Data from Blink's use counters reveals
however that the Unicode case-insensitivity is never triggered, and even
ASCII case-insensitivity is triggered extraordinarily rarely.
Additionally, the semantics of these attributes is more like an
identifier than anything else, and so case-insensitive comparison never
really made sense in the first place (it was only done for legacy
Internet Explorer compatibility). As such, we move to converge on
case-sensitive matching in all cases.

Tests incoming, will file browser bugs after merge.

[domenic]

Tests at web-platform-tests/wpt#4044 for radio button groups. I can't figure out how to test image maps. It might be possible with something really complicated (synethetic click events at some section of the page that is covered by an image map??) but given that there are no such existing tests for me to model on, I'm going to ask that we skip requiring tests for that.

[annevk]

I don't think you can test it without some kind of framework that can actually send UI events. However, you could create a manual test for it which would be nice for implementers and reviewers.

Really happy we can remove this wart by the way.

[zcorpan]

You can use elementFromPoint(), see /html/semantics/embedded-content/the-area-element/area-coords.html for an example. (If the map is associated, you get an area element; if not, you get the img.)

[domenic]

Thanks for the tip. Tests updated. Anyone approve the PR itself?

[annevk]

Oops, this PR LGTM.

[domenic]

• Tests:
  • http://w3c-test.org/html/semantics/forms/the-input-element/radio-groupname-case.html
  • http://w3c-test.org/html/semantics/embedded-content/the-img-element/usemap-casing.html
  • http://w3c-test.org/html/semantics/embedded-content/the-object-element/usemap-casing.html
• Blink: opened https://bugs.chromium.org/p/chromium/issues/detail?id=658749
• WebKit: commented at https://bugs.webkit.org/show_bug.cgi?id=90617#c21 and opened https://bugs.webkit.org/show_bug.cgi?id=163894
• Gecko: opened https://bugzilla.mozilla.org/show_bug.cgi?id=1312456
• Edge: opened https://developer.microsoft.com/en-us/microsoft-edge/platform/issues/9496749/