Acid2 — web browser rendering conformance test

Acid2 is a reference test page used to reveal flaws in browser implementations of HTML and CSS. It encouraged interoperability among layout engines and helped drive improvements in standards support.

Overview

Acid2 is a focused diagnostic web page created to reveal rendering problems in web browsers. When a browser implements the specific HTML and CSS behaviors the test exercises, the page produces a distinctive reference result; when it does not, visual errors or broken layout make the mismatch obvious. The test concentrates on particular combinations of markup, style rules and image compositing rather than measuring performance or complete standards compliance.

Technical details

The page is constructed from a compact mixture of HTML structure, CSS rules and raster images to exercise layout edge cases. Key areas tested include box model interpretations, positioning and floats, cascading and inheritance interactions, generated content, and PNG image compositing with alpha transparency. By combining these features in a single scene the test stresses how a rendering engine applies rules in concert and exposes subtle implementation bugs.

Running the test and interpreting results

To run Acid2, users open the test page in a browser and compare the rendered result to the published reference. A correct rendering is visually distinctive and easy to check; common failures include misplaced boxes, incorrect stacking, broken transparency or missing generated content. Passing the test indicates that a browser handles the specific interactions covered, but maintainers and developers should treat it as one targeted diagnostic among many.

History and impact

Acid2 emerged from the web standards community as a practical tool for encouraging vendors to fix interoperability bugs. It followed earlier compatibility efforts and preceded later, more comprehensive tests that target additional technologies. Because the test yields an immediately visible target, it proved effective in motivating improvements in several major layout engines and in helping developers verify fixes.

Limitations and successors

Although passing Acid2 became a useful indicator of modern standards support, the test covers a limited feature set and does not prove full compliance with all web specifications. Over time, new tests and benchmarks have been developed to address other layers of the platform; Acid2 remains a historical milestone in interoperability testing rather than a complete validation suite.