When you look at the OOUI demos, whenever you switch "tabs" (from "widgets" to "icons" etc) there's a brief second where the title loads -- unstylized -- and then immediately "jumps" to the structure we expect.
This is because we inject all CSS through the JS. That's fine, but we could, fairly easily, set the original content to have display:none (or even a spinner?) and when the JS is ready, unhide the content.
The "jumping" of stylized stuff is pretty jarring.