BaseURL is not specified correctly for CSS module scripts

[dminor]

What is the issue with the HTML Standard?

When constructing a CSS module script, Step 5 of the specification says "Let sheet be the result of running the steps to create a constructed CSSStyleSheet with an empty dictionary as the argument." and Step 2 of that specification says "Set sheet’s location to the base URL of the associated Document for the current global object.", but there's a web-platform test that checks that the BaseURL is the URI of the module script, not the document. The test was added here: web-platform-tests/wpt#29408.

I ran into this while implementing CSS module scripts in Firefox (See https://bugzilla.mozilla.org/show_bug.cgi?id=1720570). I've implemented the behaviour that makes the test pass, but that doesn't seem correct according to the specification. I think in this case it makes sense to update the specification to match the test behaviour.

[HITMAN5050]

“Hi — I’d like to work on investigating this. I’ll run cross-browser tests, review the spec wording around ‘base URL’ for CSS module scripts, and propose either test or spec changes. Is anyone currently working on this?”

[annevk]

Using the module script URL does seem more sensible.

cc @dandclark @whatwg/modules

[dandclark]

I agree, using the module script URL is the behavior that makes sense here.

The right thing to do is probably to have step 5 of https://html.spec.whatwg.org/#creating-a-css-module-script pass in the options dictionary {"baseURL": script's base URL }. That'll set the stylesheet base URL, which if present is used over the sheet's location in computing the style resource base URL.

I'm not sure if the location should be changed. It's reflected in the href attribute. Step 2 of create a constructed CSSStyleSheet says it should be set to the "base URL of the associated Document", but in my local testing it doesn't look like that's happening. I see new CSSStyleSheet().href returning null in Chromium, Firefox, and Safari. Not sure if the spec is wrong there or I'm just missing something.

[dminor]

Fwiw, using the options dictionary would be a nice solution implementation wise.