SELECTORS API

One of the most popular capabilities of JavaScript libraries is the ability to retrieve a number of DOM elements matching a pattern specified using CSS selectors. Indeed, the library jQuery (www.jquery.com) is built completely around the CSS selector queries of a DOM document in order to retrieve references to elements instead of using getElementById() and getElementsByTagName().

The Selectors API (www.w3.org/TR/selectors-api) was started by the W3C to specify native support for CSS queries in browsers. All JavaScript libraries implementing this feature had to do so by writing a rudimentary CSS parser and then using existing DOM methods to navigate the document and identify matching nodes. Although library developers worked tirelessly to speed up the performance of such processing, there was only so much that could be done while the code ran in JavaScript. By making this a native API, the parsing and tree navigating can be done at the browser level in a compiled language and thus tremendously increase the performance of such functionality.