Allow more characters in custom element names

[domenic]

This follows whatwg/dom#1079.

• At least two implementers are interested (and none opposed):
  • Gecko
  • Chromium
• Tests are written and can be reviewed and commented upon at:
  • TODO first implementer should do this!
• Implementation bugs are filed:
  • Chrome: https://bugs.chromium.org/p/chromium/issues/detail?id=1334640
  • Firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=1773312
  • Safari: https://bugs.webkit.org/show_bug.cgi?id=241419

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

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

[annevk]

This looks good to me. I like how this simplifies things quite a bit and reduces our reliance on ABNF and derivatives.

[annevk]

Hmm, I guess 0th is technically correct, but I wonder if we are using that consistently. Perhaps we should support string[0] to make it less confusing?

[domenic]

I doubt we are using it consistently. But the algorithms in https://infra.spec.whatwg.org/#strings use it indirectly quite a lot, e.g. "Let potentialPrefixCodeUnit be the ith code unit of potentialPrefix." where i can be 0. And the plus side is that nobody will be confused by 0th, although they might be confused by first.

Allowing string[0] is a bit tricky IMO because it doesn't clarify whether that's code point or code unit. Maybe string[0]_cu and string[0]_cp, but that feels a bit like adding notation for notation's sake...