* Defines the critical subresources for CSS as @imported style sheets, including
  indirectly imported ones. This was previously defined nowhere; probably it
  should live in CSS, but for now it lives here.
* Makes it clearer that the steps to obtain the resource are the default steps,
  which some link types override.
* Adds explicit steps for fetching critical subresources, and ties those to the
  load/error events.
* Uses fetch primitives like network error and ok status instead of vague
  language.
* Makes it clearer how incorrect Content-Types for CSS generate error events, by
  defining the new "resource-specific fetch error" concept.
* Makes it clearer that resources apply if and only if their main fetch
  succeeds. (Critical subresource fetches generate error events, but do not
  impact the "obtained successfully" or "apply" concepts.)

Fixes #3532.

This will also fix #3610