CSS selector lists have a critical behavior: if any single selector in a list is invalid or unsupported, the entire rule is discarded. The :is(), :where(), and :has() pseudo-classes solve this with 'forgiving selector' behavior — they silently ignore invalid or unsupported selectors and still apply styles to the valid ones. This makes them useful for progressive enhancement and cross-browser compatibility, in addition to their existing benefits of reducing code and controlling specificity.
