Remove more XML-derived attribute name validation

[domenic]

• Change the behavior of the dataset named setter.
• Change the conformance-related definitions for custom data attributes and custom element attributes.

Closes #11439.

• At least two implementers are interested (and none opposed):
  • Chromium confirmed below
  • Going to assume that this minor addition is OK with WebKit and Gecko per Gecko's explicit interest in Allow more characters when creating various nodes dom#1079 and @annevk's approval below.
• Tests are written and can be reviewed and commented upon at:
  • https://github.com/web-platform-tests/wpt/pull/53251/files#diff-cc0595be8cdff8311f0c29be67f13dfecfba203b2a8966b7353dfb8f341d701c
• Implementation bugs are filed:
  • I'm adding comments to all the bugs filed in Allow more characters when creating various nodes dom#1079.
• Corresponding HTML AAM & ARIA in HTML issues & PRs: N/A
• MDN issue is filed: (We did not do this for the original PRs because they were so old.)
• The top of this comment includes a clear commit message to use.

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

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/custom-elements.html ( diff )
/dom.html ( diff )
/infrastructure.html ( diff )

[annevk]

I didn't quite realize this when we made the change for custom elements, but this does mean that HTML and XHTML have now diverged in what they can express. This was already the case to some extent in various scenarios so that's probably okay.

[josepharhar]

I implemented the dataset logic in chromium already because the code for setAttribute which looks at the name validation is called by the dataset code. WPT was updated here: https://github.com/web-platform-tests/wpt/pull/53251/files#diff-cc0595be8cdff8311f0c29be67f13dfecfba203b2a8966b7353dfb8f341d701c

I'm guessing that the custom elements changes are not testable.

[domenic]

Thanks very much @josepharhar! Then, let me merge this, under the assumption we have enough Gecko and WebKit support to proceed. If that turns out to be a bad assumption, I will revert.