Acid3 test is a web test page from the Web Standards Project that checks a web browser's compliance with elements of various web standards, particularly the Document Object Model (DOM) and JavaScript.

If the test is successful, the results of the Acid3 test will display a gradually increasing fraction counter with colored rectangles in the background. The number of subtests passed will indicate the percentage that will be displayed on the screen. This percentage does not represent an actual percentage of conformance as the test does not really keep track of the subtests that were actually started (100 is assumed). Moreover, the browser also has to render the page exactly as the reference page is rendered in the same browser. Like the text of the Acid2 test, the text of the Acid3 reference rendering is not a bitmap, in order to allow for certain differences in font rendering.

Acid3 was in development from April 2007, and released on 3 March 2008. The main developer was Ian Hickson, a Google employee who also wrote the Acid2 test. Acid2 focused primarily on Cascading Style Sheets (CSS), but this third Acid test also focuses on technologies used on modern, highly interactive websites characteristic of Web 2.0, such as ECMAScript and DOM Level 2. A few subtests also concern Scalable Vector Graphics (SVG), Extensible Markup Language (XML), and data URIs. Controversially, it includes several elements from the CSS2 recommendation that were later removed in CSS2.1, but reintroduced in World Wide Web Consortium (W3C) CSS3 working drafts that have not made it to candidate recommendations yet.