Allow "style" and "json" in as attribute for modulepreload

[KurtCattiSchmidt]

Updates the definition of modulepreload to support as="style" and as="json" for preloading style and JSON module scripts respectively.

Fixes #10233

• At least two implementers are interested (and none opposed):
  • Chromium
  • …
• Tests are written and can be reviewed and commented upon at:
  • …
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/466888680
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2004751
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=303761
  • 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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[annevk]

Should we also disallow "json" for <link rel=preload>?

[KurtCattiSchmidt]

Should we also disallow "json" for <link rel=preload>?

You're right, "json" doesn't really make sense in preload if it's supported for modulepreload. I think this should be handled separately though, because WebKit recently added support for <link rel=preload as=json>, and we can have both for some window of time.

Filed #11995 to track and discuss separately.

[annevk]

I'd rather tackle them together as I have a feeling #11995 will not be addressed anytime soon otherwise. And it would be good to have that infrastructure in place as we add more types, such as text and bytes.

[KurtCattiSchmidt]

I assumed there would be some window of overlap between the new and deprecated version, which is why I filed a separate bug. But if we can do them in one shot that's great. This feature is a little different in that it's not directly observable, so it does seem safe to just deprecate.

I'll update this PR to remove <link type="preload" as="json"> today.

@rniwa - you recently added support for <link type="preload" as="json"> in WebKit so your input here is appreciated.

[KurtCattiSchmidt]

Latest push handles removing "json" for "preload". I also added the implementation bugs. I'll work on tests next.