In CSSSelectorParser::consumeRelativeScopeSelector we sometimes insert a PseudoClassType::RelativeScope pseudo-class at the front of the selector argument of a :has(). This is what ensures that the matching of the selector argument does not break out of the :has scope. But we only do this if (a) we don't already mention :scope in the selector argument, or (b) the :has() scope combinator is not DescendantSpace. But we do need this synthesized RelativeScope pseudo-class, otherwise this will alert true when it shouldn't: <div id=x><span></span></div> <script> alert(x.matches(":has(:scope span)")); </div>