A layout engine is a software that takes web content (such as HTML, XML, image files, etc) and formatting information (such as CSS, XSL, etc) and displays the formatted content on the screen. It "paints" on the content area of a window, which is displayed on a monitor or a printer. A layout engine is typically used for web browsers, email clients, or other applications that require the displaying (and editing) of web contents.

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.

See also

• List of layout engines
• Comparison of layout engines