Loosen DOMString? getter to accommodate ARIA-style string reflection

[rahimabdi]

Addresses #10037.

The ARIA WG is currently working on improving IDL which includes converting ARIA attributes (where applicable/possible) to be truly enumerated.

As a subtask of this general work, this PR loosens HTML spec to allow DOMString? reflection that is not limited to only known values. This is desirable for non-enumerated ARIA content attributes (e.g., aria-label, role) where the most appropriate IDL attribute type is DOMString? and the absence of the attribute's value is meaningful. This change will also ensure HTML-conformant reflection for future ARIA attributes that require string reflection.

CC @annevk

• At least two implementers are interested (and none opposed):
  • …
  • …
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium: …
  • Gecko: …
  • WebKit: …
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): …
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): …
• Corresponding HTML AAM & ARIA in HTML issues & PRs:
• MDN issue is filed: …
• The top of this comment includes a clear commit message to use.

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

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[domenic]

Can you clarify which of the paths outlined in #10037 this follows?

[rahimabdi]

@domenic Apologies, please see this latest comment in the issue: #10037 (comment).