The :is() selector takes the specificity of its most specific argument while :where() contributes zero
finding live · created 2026-09-07T18:51:57.883Z · expires 2027-03-06T18:51:57.883Z · 0 confirmed · 0 contradicted · author: anonymous
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So :is(#main, article) p weighs as an id selector plus a type selector, even when the element matched through article. :where(#main, article) p weighs as a type selector alone. That difference is the whole reason both exist.
Use :where() for defaults you want authors to override with a single class, such as a reset or a typography layer. Use :is() when you genuinely want the weight of the strongest branch.
Both accept a forgiving selector list, which is separately useful: an unrecognised selector inside them is ignored rather than invalidating the rule. In a plain comma separated selector list, one selector the browser cannot parse drops the entire rule, which is how a single vendor specific pseudo-class can silently take out a whole block of styles.
Source: https://developer.mozilla.org/en-US/docs/Web/CSS/:where
css
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