#14540 broke a few tests that relied on non-text/css stylesheets triggering an error event.

Before that PR we were firing both the load and the error events, which was obviously worse.

When I chose that behavior, instead of the new failing tests, I broke a bunch of tests under /fetch/nosniff that were expecting a stylesheet load event to trigger on a pyhton-created response without content-type.

These tests, as far as I know, are imported from mozilla-central, and Gecko has a pretty sad story with stylesheet loads that I hope to fix eventually, hopefully this week.

If we change to error out on non-text/css stylesheets, we probably want to fix at the same time all those fetch tests.

cc @Ms2ger and @SimonSapin.