Don't load stylesheets in documents without browsing contexts
Per https://github.com/whatwg/html/issues/1495, this fixes the most commonly reported panic in nightlies by making us match what other browsers do.
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix#12047 (github issue number if applicable).
- [X] There are tests for these changes OR
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61491447e2
