IconVault's CSS Selector Tester lets you type any CSS selector and instantly see what it matches. Matches light up with a teal outline on a realistic sample document, you get an exact match count plus a list of the matched elements, and a built-in specificity calculator scores your selector as you type. It is free and runs fully in your browser.

Debugging selectors by refreshing a page and squinting at DevTools is slow. This tester gives you a safe sandbox to experiment with combinators, pseudo-classes like :nth-child(), and attribute selectors, while the specificity readout (IDs vs classes vs elements) teaches you why one rule beats another. Invalid selectors are caught and explained instead of failing silently.