Avoiding a flash of unstyled elements

Complex elements are hidden with CSS until they set the ui-ready attribute after their first render:

ui-select:not([ui-ready]) { visibility: hidden; }

Checklist:

1. Load ui.css in <head> (render-blocking).
2. Load ui.js with defer.
3. Reserve space for tall elements with min-height if layout shift matters.
4. Add the theme prelude from Getting started.