A web browser engine, (sometimes called layout engine or rendering engine), is a software component that takes marked up 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 web browser engine is typically embedded in web browsers, e-mail clients, on-line help systems or other applications that require the displaying (and editing) of web content.

The first web browsers were monolithic. Later they adopted a more modular approach and were split into a user interface and an engine.

The engine does most of the work. It essentially takes a URL and a screen rectangle as arguments. It then retrieves the document corresponding to the URL and paints a graphical representation of it on the given rectangle. It handles links, cookies, scripting, plug-ins loading and other matters.

The user interface provides the menu bar, address bar, status bar, bookmark manager, history and preferences window among other things. It embeds the engine and serves as an interface between the user and the engine. Since it provides the graphical elements surrounding the area in which the engine paints documents, the term chrome(not Google's Chrome)^{[1] [2]} is sometimes used to refer to it.

The advantage of this modular approach is that it then becomes easy to embed web browser engines in a variety of applications. For example, the same engine used by a web browser can be used by an email client to display HTML mail. On-line help systems integrated in applications have largely moved from using custom formats to using standard HTML displayed with a web browser engine.

Gecko, the Mozilla project's open-source web browser engine, is used by a variety of products derived from the Mozilla code base, including the Firefox web browser, the Thunderbird e-mail client, and SeaMonkey internet suite.

Trident, the web browser engine from Internet Explorer, is used by many applications on the Microsoft Windows platform, such as netSmart, Outlook Express, some versions of Microsoft Outlook, and the mini-browsers in Winamp and RealPlayer.

Opera Software's proprietary Presto engine is licensed to a number of other software vendors, and is used in Opera's own web browser.

KDE's open-source KHTML engine is used in KDE's Konqueror web browser and was the basis for WebKit, the rendering engine in Apple's Safari and Google's Chrome web browsers.

• Comparison of web browser engines
• List of web browser engines
• Layout (computing)
• Rasterisation
• Page description language