Removing blocking=render from "implicitly render-blocking" elements unblocks rendering

[xiaochengh]

We currently have some elements that are "implicitly render-blocking" (not a formally spec-ed term), namely, parser-inserted stylesheet link elements, parser-inserted style elements and synchronous scripts. For backward compatibility, we block rendering for them even if they do not carry the blocking=render attribute.

The current spec also says:

An element is render-blocking if its blocking tokens set contains "render".

Whenever a render-blocking element el becomes browsing-context disconnected, or el's blocking attribute's value is changed so that el is no longer render-blocking, then unblock rendering on el.

This means, e.g., if we have a parser-inserted <link blocking=render rel=stylesheet> and we remove blocking=render from it while it's still loading, we should unblock rendering for it. This is not a desired behavior.

I'll put up a PR to fix it.

@domenic

[domenic]

This is wrong and breaks backward compatibility.

I'm not sure if it breaks backward compatibility. A page used a new feature (blocking=render) and got a new behavior (dynamic render-blockingness that can change over time).

Now, it might be the case that that new behavior is not desired. But I don't think it's a compat issue.

Do you agree?

[xiaochengh]

That makes sense. I'll update the issue description.