Remove duplicate script-blocking style sheet requirement

[domfarolino]

Per #3103 (comment), this PR removes a reference to HTML's "script-blocking style sheet" concept from https://drafts.csswg.org/cssom/#requirements-on-user-agents-implementing-the-xml-stylesheet-processing-instruction.

PTAL @annevk. Btw do you have any opinions on what we should do about the same reference to the same term at the bottom of https://drafts.csswg.org/cssom/#requirements-on-user-agents-implementing-the-http-link-header? These days HTML defines the Link header processing model, and as per https://html.spec.whatwg.org/#table-link-relations we only support preconnect and preload, not what CSSOM-1 defines here (and this is noted by the section's "Note yet widely implemented" call-out). IMO it seems best to expand this PR to also remove (at the very least, scoped to this PR) that reference from that section and let HTML work it out if it chooses to.

[annevk]

We cannot remove this text without equivalent text that style and <link rel=stylesheet> have (which also seems like it could be abstracted more, but I haven't delved into it too deeply):

If element contributes a script-blocking style sheet, append element to its node document's script-blocking style sheet set.

I think the Link header part can be removed in its entirety. I would do that in a separate PR that references whatwg/html#8741.

[domfarolino]

Sure, that sounds good. Do you have a preference as to whether that text should be inside the algorithm steps (I guess close to the end like in HTML) or as this spec had it, outside the algorithm in a normative block below?

[annevk]

I don't really. I kinda hope it can be abstracted more at some point so we don't need to state this for each type.

[domfarolino]

That sounds ideal. If it can happen outside of this PR (if that's what you're suggesting), then would you mind reviewing this for now? (I can't formally add anyone as a reviewer I guess...)

[annevk]

As clarified via chat this is blocked on the new changes being pushed.

[domfarolino]

This is still used by the Link section, so we can probably remove it once we address #8759 (comment).

[domfarolino]

OK, all set for review @annevk.

[annevk]

This doesn't look great I'd say, but I guess it's an improvement.

[domfarolino]

Yeah, I kinda agree. Mostly my priority here is closing out #3103 since it seems bad to have duplicate out-of-sync definitions of the same criteria for script-blocking-ness. Is there anything you'd prefer me to do in this PR to make this all more satisfiable? I guess it's probably better to modify the processing model directly to append the style sheet to the script-blocking style sheet set before we fetch, and the in the response handler remove it from the set etc., but I'm not sure if that matches what you think would be the ideal long-term state for all of this anyways given #8759 (comment).

[annevk]

Looking at everyone that invokes "create a CSS style sheet" it seems to me this could be added to "add a CSS style sheet" and removed from all the callers of "create a CSS style sheet".

Those algorithms could also use a lot of cleanup in terms of making their arguments more explicit. And not having this weird "environment encoding" callback setup.

Also, https://html.spec.whatwg.org/#contributes-a-script-blocking-style-sheet has to be cleaned up as it doesn't account for <?xml-stylesheet?> at the moment.

[domfarolino]

I've added the check to "add a CSS style sheet". If you think this looks good I'll make PRs to HTML and https://w3c.github.io/webvtt/#ref-for-create-a-css-style-sheet to remove the reference to the "contributes ..." condition since it appears inside "add", which indeed is called by "create".

[annevk]

Yeah this looks okay.

[annevk]

I can merge this, but a current editor should probably do a final review. @emilio?

[emilio]

Looks good, thanks!