[@sheet] Fragment-only identifiers (without a URL) should allow inline `@sheet` references on the same document to be included globally (even within shadow roots).

Fragment-only identifiers (without a URL) should allow inline `@sheet` references on the same document to be included globally (even within shadow roots). This wasn't brought up in the CSSWG discussions at all, but is important for DSD without requiring an external file (to avoid FOUC).