<meta name="color-scheme">
Announces which color schemes the page knows how to behave in — before the CSS arrives.
It declares which schemes the document can live with. With content="light dark", the browser may paint the default background, the form controls and the scrollbars in the variant the system asks for — and the white flash before the CSS lands, dark mode's resident ghost, disappears.
It mirrors the CSS color-scheme property, but wins the race by being read during the <head> parse. The caveat: declaring dark without styles to match yields dark form controls on a light page — the meta is a promise the CSS must keep.
<meta name="color-scheme" content="light dark">