Chrome uses the stylesheet URL after stripping the query part.
Edge uses the document URL.
Firefox uses the stylesheet URL after stripping the query part.
Safari uses the document URL.

I can't figure out how subresource requests from stylesheets hook into Fetch. The closest I can find for background-image is https://drafts.csswg.org/css-backgrounds-3/#background-image which leads to https://www.w3.org/TR/CSS21/syndata.html#value-def-uri which doesn't seem to hook into Fetch.

I can't find WPT for the referrer of subresource requests from stylesheets without service workers. We should probably figure that out first then add this service worker test.

Since it doesn't seem specced and browsers disagree, we probably should move the referrer tests to a tentative wpt. See https://github.com/w3c/csswg-drafts/issues/904 and https://github.com/w3c/csswg-drafts/issues/562 (thanks to astearns at https://freenode.logbot.info/whatwg/20181218#c1881201)

Note that later annevk confirmed "style sheet URL is the referrer". Either way these assertions should be in a different promise_test so we still get test coverage on base URL even if referrer is wrong.