Style matching tries a third of the rules, queries prepare their selector once, sibling walks stay fast

To style an element the engine only tries the rules whose last part could match it: rules for its tag, its classes or its id. A rule whose last part had none of those was tried against every element on the page. A code-hosting page has over a thousand such rules - hundreds of theme blocks written for the page root, rules keyed on an attribute such as a colour mode, and design-system rules wrapped in :where() - so every full restyle tried three million rules to make twenty-six thousand matches. Rules for the page root now go to the root element only, attribute rules to elements carrying that attribute, and :where(.x) and :is(.x) rules to elements with class x. The matcher still decides every match; it is simply asked a third as often.

querySelectorAll already prepared its selector once for the whole search. querySelector, matches and closest split the selector list and read every part again for each element they tested. All four now share one prepared form, kept for the next call with the same selector, with simple selectors such as a class or a tag compared directly.

nextSibling and previousSibling found an element's position by scanning its parent's children on every read, so walking a list of n items cost n times n steps. The position of the last answer is now remembered and checked first, so each step of a walk is one comparison. The most read node links and names - parent, children, siblings, nodeName, tagName, id and className - are also answered directly from the document instead of through the full member lookup, and every such answer can be checked against the full lookup with a switch.