Flash of unstyled content

Content painting before its stylesheet has been applied, usually because the CSS was loaded asynchronously or injected by script. It looks like a broken page even though nothing is broken, and it is worse on a slow connection where the unstyled state persists long enough to read. The related font versions — invisible text while a webfont loads, or a visible swap once it arrives — come from the same cause and are controlled with `font-display`.