A browser engine is not a stand-alone computer program but rather a critical piece of a larger program, such as a web browser, from which the term is derived. The word "engine" is an analogy to the engine of a car.

Besides "browser engine", two other terms are in everyday use regarding related concepts: "layout engine" and "rendering engine".^[1][2][3] In theory, layout and rendering (or "painting") could be handled by different engines. In practice, however, a given browser engine's rendering and layout components are tightly coupled and rarely encountered on their own outside the browser engine, unlike the case with browsers' scripting engines.

In addition to layout and rendering, a browser engine enforces the security policy between documents, handles navigation through hyperlinks and data submitted through forms, and implements the Document Object Model (DOM) exposed to scripts associated with the document.

Every significant web browser supports JavaScript (JS) to enable a wide range of interactive behavior specified by the publisher. However, JS language support is treated as a separate concern, and browser vendors use a dedicated JavaScript engine to parse and evaluate it.^[4] Although JS was initially created as a simple language for use on web pages, it became a general-purpose language used elsewhere. Browsers have sophisticated methods of coordinating their engines.^[1][5]

Browser engines are used in other types of programs besides web browsers. For example, an email client might rely on a browser engine for displaying HTML email. The Electron framework, powered by Blink and V8 from Google's Chromium project, enables software developers to use technologies first created for the web platform—such as HTML, CSS, JavaScript, and the DOM—to create graphical desktop applications. Developer familiarity browser-based technologies has resulted in Electron and other cross-platform, browser engine-based application frameworks often being used in lieu of traditional GUI toolkits.

The layout of a web page is typically specified by Cascading Style Sheets (CSS). Each style sheet is a series of rules which the browser engine interprets to determine how the document should be presented. For example, a published web page might use CSS to specify rules to style typography details, such as font, color, and text size. The engine combines all relevant CSS rules to calculate the concrete graphical coordinates for the visual representation it will paint on the screen.^[1][5]

Some engines may begin rendering before a page's resources are downloaded. This can result in visual changes as more data is received, such as images being gradually filled in or a flash of unstyled content.

• Apple created the WebKit engine for its Safari browser by forking the KHTML engine of the KDE project.^[6] All browsers for iOS must use WebKit as their engine.^[7]
• Google originally used WebKit for its Chrome browser but eventually forked it to create the Blink engine.^[8] All Chromium-based browsers use Blink, as do applications built with CEF, Electron, or any other framework that embeds Chromium.
• Microsoft has two proprietary engines, Trident and EdgeHTML. Trident is used in the Internet Explorer browser. EdgeHTML was the original engine of the Edge browser, but that was remade with the Blink engine.^[9] EdgeHTML is still used in some UWP apps.^[10]
• Mozilla develops the Gecko engine for its Firefox browser and the Thunderbird email client.^[2]

Timeline edit

Only the duration of active development is shown, which is when relevant new Web standards continue to be added to the engine.

• Comparison of browser engines