Intent to Ship: Parser changes to "select"

[Keith Cirkel]

Summary: Support for additional elements inside the select element.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1933591

Specification: https://github.com/whatwg/html/pull/10557

Standards Body: WHATWG

Platform coverage: All

Preference: dom.element.popoverhint.enabled

Devtools bug: N/A

Other browsers:

  - Blink: Shipping around M135.

  - WebKit: Shipping in Safari TP 242 (https://webkit.org/blog/17934/release-notes-for-safari-technology-preview-242/).

web-platform-tests: https://wpt.fyi/results/html/semantics/forms/the-select-element/customizable-select/select-parsing.html?label=experimental&label=master&aligned

As part of "customizable select", all browsers have needed to make changes to the HTML parser in order to drop special modes around parsing `<select>` elements and its children. We've had this behaviour _flagged_ in nightly for a release cycle.

I've landed https://phabricator.services.mozilla.com/D295502 as part of the 153 train, which enables the new parser modes by default. This is a prerequisite for supporting new elements such as `<selectedcontent>` inside of `<select>`, as well as the new `appearance:base` mode for `<select>`. I'll be following up by cleaning up the feature flag. 

As usual, if any issues arise, please let the DOM team know and we can roll this back. We do not anticipate issues given the 152 nightly cycle had this unflagged, but this means that from 153 this will be on in release channels. 

If anyone has any concerns please let me know!

Happy browsing.

[Keith Cirkel]

Errata: the flag is `dom.lift_select_parser_restrictions.enabled`, not `dom.element.popoverhint.enabled` as I accidentally copy/pasted in OP