Acid2 is a webpage that test web browsers' functionality in displaying aspects of HTML markup, CSS 2.1 styling, PNG images, and data URIs.

The Acid2 test page will be displayed correctly in any application that follows the World Wide Web Consortium and Internet Engineering Task Force specifications for these technologies.

With the release of Internet Explorer 8 on 19 March 2009, the latest versions of all major desktop web browsers now pass the test.

Acid2 was first proposed by Håkon Wium Lie, chief technical officer of Opera Software and creator of the widely used Cascading Style Sheets web standard.

[1] In a 16 March 2005 article on CNET, Lie expressed dismay that Microsoft Internet Explorer did not properly support web standards and hence was not completely interoperable with other browsers.

The original Acid1 test had forced browser makers to fix their applications or face embarrassment; Lie hoped that Acid2 would do the same.

In browsers that do not implement SGML-style comments, the original test displayed the word "ERROR" on the bottom part of the face.

This zone is active for pages loaded via UNC paths, named addresses without dots (like http://mysite/), and sites that bypass the proxy settings.

Actions such as changing font sizes, zoom level, and applying user stylesheets can break the display of the test.

[38] The following browser settings and user actions invalidate the test:[38][39] If rendered correctly, Acid2 will appear as a smiley face below the text "Hello World!"