A web browser engine, also called a rendering or layout engine, is the part of a browser that turns web page source into something you can see. Mozilla’s developer documentation defines a rendering engine as software that “transforms HTML, CSS, and other resources of a web page into a visual representation on a screen.” It handles parsing the markup, applying styles, computing layout, and painting the result.
The engine is distinct from the browser itself. As the WebKit project puts it, “WebKit is an engine, not a browser.” WebKit describes itself as “an open source Web content engine for browsers and other applications” focused on “content deployed on the World Wide Web, using standards-based technologies such as HTML, CSS, JavaScript and DOM.” A browser wraps an engine with a user interface, networking, bookmarks, and other features.
Several engines have dominated at different times. Mozilla lists four major ones: Blink, “used in Chrome and Edge”; Gecko, “Mozilla’s engine used in Firefox”; WebKit, “used in Safari”; and Trident, the legacy engine “formerly used in Internet Explorer.” Because the engine decides how standards are interpreted, which engines are popular has a large effect on what web developers can rely on.
Understanding the engine layer explains much of web history. The browser wars were, underneath the branding, contests between rendering engines, and modern debates about web standards and compatibility are largely debates about how these engines behave.