It appears that in the attached test-case, a FOUC is being caused by the <input type=hidden> in the <head> of the document. The delay is more noticeable if the stylesheet takes longer to download, but doesn't seem to be tied to the specific stylesheet I've used in the test-case (I tried a stylesheet from the BBC site, among others, and they all caused a flash relative to the time it took to fetch the remote CSS file).
Bug 1520328 Comment 0 Edit History
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It appears that in the attached test-case, a FOUC (flash of unstyled content) is being caused by the <input type=hidden> in the <head> of the document. The delay is more noticeable if the stylesheet takes longer to download, but doesn't seem to be tied to the specific stylesheet I've used in the test-case (I tried a stylesheet from the BBC site, among others, and they all caused a flash relative to the time it took to fetch the remote CSS file).