Acid2 is a test suite published and promoted by the Web Standards Project to identify web page rendering flaws in browsers and authoring tools. Acid2 was released on April 12, 2005. It has been developed in the spirit of the Acid1 test from 1998. The Acid tests test many features on a single page and report test results graphically.

Acid2 tests features of HTML and, more prominently, CSS. The function of testing such features is to highlight the problems with browsers that do not display it correctly. The Acid2 test should render correctly on any browser that follows the W3C HTML and CSS 2.0 specifications. Because Acid2 tests how web browsers deal with faulty code, the test is intentionally not written to W3C CSS standard specifications, and fails validation. This is expected and was the intention of its designers.

Safari was the first web browser to pass the test, with the passing version 2.02 released on October 31, 2005.

Overview of standards tested

Acid2 tests the following web standards:

Alpha transparency on PNG images – the eyes are transparent PNGs

The object element

Absolute, relative and fixed positioning using CSS

The CSS box model

CSS tables

CSS margins

CSS generated content

CSS parsing – Acid2 includes a number of illegal CSS statements to test error handling

Paint order

CSS line heights

Hovering effects

Because Acid2 is not a comprehensive test, it does not guarantee total conformance with any specific standard.

The Web Standards Project has created a special version of Acid2 test, because the data: URIs used in the original test have never been formally standardized.

Compliant applications

Acid2 as rendered by compliant applications

If rendered correctly, Acid2 will appear as a smiley face below the text "Hello World!" in the user's browser, with the nose turning blue when the mouse cursor hovers over it. At the time of the test's release, no browsers could render Acid2 correctly, but now a number of applications pass the test by rendering it correctly:

Officially released

WebKit-based applications

Safari, the web browser included in Mac OS X

OmniWeb, a web browser for Mac OS X

Shiira, a web browser for Mac OS X

iCab, a web browser for Mac OS X

Konqueror, a web browser for KDE

Prince, an XML-to-PDF converter for Windows and Linux

Presto-based browsers

Opera, a web browser for Windows, Mac OS X, Linux, and BSD

Internet Channel, a version of the Opera browser for the Nintendo Wii game console.

In development

Gecko 1.9 based applications

Mozilla Firefox 3, a web browser for Windows, Mac OS X, and Linux

Mozilla Prism, a web application platform for Windows, Mac OS X and Linux

Tkhtml Html Viewer 3, a web browser for Windows and Linux

Non-compliant applications

Although Internet Explorer has also been moving towards better CSS compliance, the current version of Internet Explorer, Internet Explorer 7, does not pass the test. Chris Wilson, the Internet Explorer Platform Architect, described Acid2 as a "wish list" of features instead of a true test of standards compliance. Nonetheless, Internet Explorer 8 will include a new rendering mode known as "IE8 standards mode". IE8 standards mode is not enabled by default, but can be triggered by inserting a special flag into a web page. In IE8 standards mode, IE8 may be able to render the Acid2 graphics, but since Acid2 does not trigger IE8 standards mode, IE8 does not pass the test.

Browsers based on the current version of the Gecko layout engine, such as Firefox, Camino, and SeaMonkey, do not pass. However, Acid2 support is planned for Gecko 1.9, and so these browsers are expected to pass Acid2 once Gecko version 1.9 is finished. This includes Mozilla Firefox 3, and Mozilla Firefox 3 Beta 2 passes Acid2 test successfully.

The PlayStation 3 web browser, based on NetFront, also fails the test.

Passing conditions

A passing score is only considered valid if the browser's default settings were used. Changing font sizes, zoom level, applying user stylesheets, etc. can break the display of the test. This is expected and is not relevant to a browser's compliance.

Ian Hickson, the author of the test, has provided WaSP with clarifications about the Acid2 test and how things should behave.

The following browser settings and user actions may affect the rendering of Acid2 page without resembleing on the browser's compliance:

Scrolling. A part of the face remains fixed when you scroll.

Zooming to any level other than 100% (default).

Disabling images.

Opera's Fit to width and SSR (Small Screen Rendering) modes. These are off by default, and the test is not valid when they are enabled.

User colors, fonts etc.

User style sheets, Opera's user mode for styling.

User JavaScripts.

Timeline of compliant and partially compliant browsers

The Acid2 test was announced by Håkon Wium Lie on March 16, 2005. The test was coded by Ian Hickson in collaboration with the Web Standards Project and the larger web community. The test was officially released on April 13, 2005. On April 23, 2005, Acid2 was updated to fix a bug.

The following is a list of releases noting significant builds of applications that passed the test.