https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/V...
I used these CSS styles in the late 90s to make internal browser-based apps for our call centre agents that looked & felt more like native Windows desktop apps.
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Shameless plug:
In neutrino (my webview wrapper), we grab those and inject them as prefixed CSS variables (--neutrino-NameOfSystemColor).
Works on windows/macos/gtk/qt, and except for qt it works live (if the user changes the accent tone, the CSS variable updates automatically).
https://alganet.dev/neutrino/ Screenshot of Mint: https://imgur.com/a/WyjpwsR
These were very handy in simpler times but I definitely see why we don't want them around these days.
That's also why many browsers also don't allow popups without an URL bar now.
For some time, Internet Explorer even had chromeless popups (no OS decorations, title bar or anything). Absolutely nasty, you could draw anything.