Add external <script type=speculationrules>

[vickiez]

Add external <script type="speculationrules">

Allows a speculation rules script element to have a src pointing to a
file of speculation rules, enabling developers to easily reuse a set of
speculation rules across HTML documents.

WICG/nav-speculation#348

Spec changes:

• Add a row for external speculation rules in the attributes table of
  4.12.1 The script element

• During prepare the script element, handle the case where src
  content attribute is present and type is "speculationrules"

• Define a "fetch speculation rules" algorithm to be used by both the
  external script and 7.6.3 The Speculation-Rules header sections.

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• At least two implementers are interested (and none opposed):
  • …
  • …
• Tests are written and can be reviewed and commented upon at:
  • https://github.com/web-platform-tests/wpt/blob/master/speculation-rules/external-speculation-rules-errors.html
  • CL with test update: https://chromium-review.googlesource.com/c/chromium/src/+/7034064/9/third_party/blink/web_tests/external/wpt/speculation-rules/external-speculation-rules.html
• Implementation bugs are filed:
  • Chromium: https://issues.chromium.org/issues/40170951
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=2008350
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=305428
  • 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: Add external <script type=speculationrules> mdn/mdn#789
• The top of this comment includes a clear commit message to use.

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

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/scripting.html ( diff )
/speculative-loading.html ( diff )
/webappapis.html ( diff )

[noamr]

Thanks for working on this! See initial comments.

[noamr]

This destination needs to be defined in the fetch spec.

[vickiez]

There's a change in progress here: whatwg/fetch#1841. Happy to help with that one, is there a proper order for merging the changes across specs?

[domfarolino]

Merging them as close together as possible provided they are both unblocked and generally agreed upon is fine. But regarding this PR: is there cross-browser interest in this feature overall? The PR template is pretty scant.

[vickiez]

Filled in the PR template with implementation bugs and just created position requests here:
Support external <script type=speculationrules> · Issue #1343 · mozilla/standards-positions
Support external <script type=speculationrules> · Issue #604 · WebKit/standards-positions

[vickiez]

Also for some context - this change was meant to be included in the base speculation rules spec change to achieve completeness. I was planning on merging it into Domenic's branch, but his change was merged into HTML first. See the comment here - #11123 (comment)

[domfarolino]

See #11697 (comment), but just so the sentiment doesn't get lost in that thread, let me ask here: is there cross-browser support for this? I'm asking because the PR template is a bit bare and I'm not sure where other folks stand on this PR, although I'm happy to review it.