From implementation perspective, this may be hard to realize because we need to know which environment can change to satisfy or not satisfy a media value. We also need to be future proof about this.

Is there a precedence in the spec for re-checking media attribute if the environment changes? IMHO the ideal think would be to define specific points at which the media attribute is checked (e.g. for rel=expect, when the link element is parsed/encountered) and make the decision whether it applies at that time.

That's how all existing media attributes work? E.g. viewport width media queries, dark mode...

This is not different fromMediaQueryList change event (https://developer.mozilla.org/en-US/docs/Web/API/MediaQueryList/change_event). Preload links are a precedence you can probably copy from.

That's how all existing media attributes work?

I'm trying to find a place where that's spelled out in the spec, do you have a link?

The best I can find in rel=stylesheet fetch setup steps is

If el'smedia attribute's valuematches the environment and el ispotentially render-blocking, thenblock rendering on el.

The only other place I can see is inhttps://html.spec.whatwg.org/#processing-the-media-attribute (which btw, needs to updated in this PR):

However, if the link is anexternal resource link, then themedia attribute is prescriptive. The user agent must apply the external resource when themedia attribute's valuematches the environment and the other relevant conditions apply, and must not apply it otherwise.

This is a bit ambiguous. One can read that as "check if media matches the environment when you're about to apply the resource". I'm not sure the interpretation of "anytime environment changes, recheck all media attributes" is clear here. What is the correct reading of this?

Preload links are a precedence you can probably copy from.

Likewise here, the only relevant text I can find is inhttps://html.spec.whatwg.org/#link-type-preload

When themedia attribute of thelink element of anexternal resource link that is alreadybrowsing-context connected, but was previously not obtained due to themedia attribute notmatching the environment, is changed or removed.

This says

When the media attribute [...] is changed or removed

not when the environment itself changes.

Removed it as a requirement and added a note instead

I guess this is just a victim of the general under-specification of link processing, indeed..

I don't understand the new note though. The normative text says that whenmedia changes, ifel'smedia doesn't match the environment, then unblock rendering onel. Which is the opposite of what the note says.

The note is about the environment changing without media changing.

For stylesheets, the handling of environment changes for themedia attribute should be defined in CSSOM. I'm not sure it actually is, but HTML forwards it to CSSOM.
https://html.spec.whatwg.org/multipage/links.html#link-type-stylesheet%3Aconcept-css-style-sheet-media

CSSOM View defines when thechange event should fire forMediaQueryList (which is separate) here:https://drafts.csswg.org/cssom-view-1/#evaluate-media-queries-and-report-changes

I'm unsure where this discussion leaves us but I personally am content with what's currently written because this area feels like an existing problem.@zcorpan, if you have any concerns or concrete suggestions, please let us know.