Add speculation rules prefetch

[domenic]

Other spec PRs that should be merged shortly after this one:

• CSP integration pull request: Integrate with speculation rules w3c/webappsec-csp#776
• Fetch integration pull request: Add the "speculationrules" destination fetch#1841
• WICG/nav-speculation pull request to rebase on top of this: Rebase on top of HTML WICG/nav-speculation#398

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This ports the relevant text from https://github.com/WICG/nav-speculation/blob/90ccea636881ab7c9b4aabd04bfa233ef33477ee/speculation-rules.bs, with minor improvements and additions. It does not port the actual navigational prefetching algorithms, which are left in https://wicg.github.io/nav-speculation/prefetch.html for now.

Part of #11123.

Notable changes from that document:

• Allows treating prerender rules as prefetch rules.

• Allows the presence of target_hint (a prerender-specific speculation rule key), but ignores it.

• Follows the precedent set by import maps a bit more closely in how it integrates with the script processing model. Normatively, it fires error events on the global object for the few things that cause complete parsing failures. (Chromium recently changed to do this.)

• Adds conformance requirements for speculation rules JSON documents, following the precedent set by import maps.

• Adds explicit callouts for when the implementation may report warnings for parsing failures, following the precedent set by import maps.

• Fixes a few minor bugs in the parsing algorithms. E.g., the previous text did not correctly deal with how CSS selector parsing returned a list, or called the slightly wrong algorithm for parsing URL patterns.

• Simplified the processing model for the Speculation-Rules HTTP header. The previous text added various concepts around pending fetches and periodic implementation-defined processing of those fetches, all designed to allow the implementation to de-prioritize these fetches in favor of other things that happen during loading. This version instead explicitly allows an implementation-defined wait time, uses a separate task source which user agents can deprioritize, and explicitly excludes non-top-level-traversables from caring about the header.

• Be explicit about when to consider speculative loads, instead of using vague language about "whenever something relevant changes".

• Give an explicit algorithm for how speculative load candidates are grouped and redundant ones coalesced, instead of stating that it must be done somehow.

• Add re-parsing of <script type=speculationrules> on base URL changes, to match the Chromium implementation.

• Remove the re-parsing of <script type=speculationrules> on text content changes, to match the Chromium implementation and also match <script type=importmap>.

• Slightly shorten some of the security and privacy considerations, e.g., by not mentioning mixed content concerns.

• At least two implementers are interested (and none opposed):
  • Chromium
  • Gecko
  • WebKit is in support of a subset of the functionality here
• Tests are written and can be reviewed and commented upon at:
  • https://wpt.fyi/results/speculation-rules (includes updates vs. original spec mentioned above)
• Implementation bugs are filed:
  • Chromium: N/A
  • Gecko: https://bugzilla.mozilla.org/show_bug.cgi?id=1969396
  • WebKit: https://bugs.webkit.org/show_bug.cgi?id=295193
  • Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): N/A
  • Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): N/A
• Corresponding HTML AAM & ARIA in HTML issues & PRs: N/A
• MDN issue is filed: N/A, MDN already documents this well
• The top of this comment includes a clear commit message to use.

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

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/acknowledgements.html ( diff )
/browsers.html ( diff )
/browsing-the-web.html ( diff )
/document-lifecycle.html ( diff )
/document-sequences.html ( diff )
/iana.html ( diff )
/index.html ( diff )
/indices.html ( diff )
/infrastructure.html ( diff )
/interaction.html ( diff )
/links.html ( diff )
/nav-history-apis.html ( diff )
/references.html ( diff )
/scripting.html ( diff )
/semantics.html ( diff )
/urls-and-fetching.html ( diff )
/webappapis.html ( diff )
/speculative-loading.html ( diff )

[domenic]

The spec text here is done and ready for review!

Marking "do not merge yet" until I get the other prerequisites listed in the OP done (mostly PRs against other specs).

[domenic]

This is now fully ready for review, including all prerequisites. I'll keep "do not merge yet" to reflect that we need to get #11442 reviewed and merged first, but that's a pretty small change...

[domfarolino]

Provided that the synchronous event is fired during post-connection steps, not just insertion steps! Not sure if that's worth a mention, probably not, but I figured I'd just beat the dead horse while I'm here.

[domenic]

Alright, I think this is ready to merge! I'll give folks another 24 hours or so to comment before doing so, but am excited to get this merged (as well as the CSP and Fetch PRs). Then I'll move on to working on the prefetch spec port.