WPT details: https://wpt.fyi/results/css/css-conditional/js/CSS-supports-selector-detecting-invalid-in-forgiving-argument.html https://wpt.fyi/results/css/css-conditional/at-supports-selector-detecting-invalid-in-forgiving-argument.html
Some functional selectors are parsed forgivingly. (e.g. :is(), :where()) If an argument of the functional selectors is unknown or invalid, the argument is dropped but the selector itself is not invalidated. To provide a way of detecting the unknown or invalid arguments in those functional selectors, this feature applies the CSS Working Group issue resolution: - @supports uses non-forgiving parsing for all selectors (https://github.com/w3c/csswg-drafts/issues/7280#issuecomment-1143852187)