A web browser's layout engine takes content (HTML, XML, images, etc.) and formatting information (CSS, etc.) and computes a visual representation of the web page, usually for output on either a monitor or a printer.

All web browsers necessarily include some form of layout engine. However, the term "layout engine" only reached popular usage when the Mozilla project designed its web browser's layout engine (Gecko) as a component that was separable from the browser. In other words, the Mozilla layout engine was reusable for web browsers besides Mozilla, and so people began to refer to Gecko as a distinct "layout engine" rather than merely a part of the web browser.

Notable layout engines include the following:

• Trident, the Internet Explorer for Windows engine
• Gecko, used in the Mozilla Suite, Firefox and various other browsers such as Galeon
• KHTML/WebCore, the Konqueror/Safari engine
• Presto, the Opera engine
• Tasman, the Internet Explorer for Mac engine
• gzilla , Dillo's rendering engine
• GtkHTML
• Links layout engine