Let the author opt-in to follow the preferred color scheme when rendering UI elements such as default colors of form controls and scrollbars as well as the used values of the CSS system colors.
When rendering web sites in a different color-scheme, e.g. a dark color scheme, the author typically applies dark styling having @media (prefers-color-scheme: dark). The author can ask tell the UA to render UI controls accordningly. Otherwise the author would have had to do their own style of these controls to match a dark theme.
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