Using "Check for issues" on https://fetch.spec.whatwg.org/ or https://url.spec.whatwg.org/ on a relatively modern MacBook Pro is a rather bad experience as it takes a long time to check the 30-15k nodes. Note that it also takes a long time for the progress bar to even appear, making you think the tool hanged or some such.

The experience would be better if it was done incrementally somehow.

Thanks for the report!

I can see the issue on my machine, Win 10.

STR

1. Load https://fetch.spec.whatwg.org/
2. Open DevTools and select the Accessibility panel
3. Pick Check for issues: "All Issues" (28K+ nodes)

There is "Initializing" text" displayed for a while and it takes dozens of seconds to get a progress bar, which takes another minute/s to finish

Honza