Browser engine

A browser engine (also known as a layout engine or rendering engine) is a core software component of every major web browser. The primary job of a browser engine is to transform HTML documents and other resources of a web page into an interactive visual representation on a user's device.

Name and scope

A browser engine is not a stand-alone computer program but 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 common use regarding related concepts: "layout engine" and "rendering engine".^[1]^[2]^[3] In theory, layout and rendering (or "painting") could be handled by separate engines. In practice, however, they are tightly coupled and rarely considered separately.

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) data structure exposed to page scripts.

Executing JavaScript (JS) code is a separate matter, however, as every major web browser uses a dedicated engine for this. The JS language was originally created for use in browsers, but it is now used elsewhere, too, so the implementation of JS engines is decoupled from browser engines. In a web browser, the two engines work in concert via the shared DOM data structure.

Browser engines are used in other types of programs besides web browsers. Email clients need them to display HTML email. The Electron framework, which is powered by the two engines of the Google Chrome browser, has been used to create many applications.

Layout and rendering

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. For example, some rules specify typography details, such as font, color, and text size. The engine combines all relevant CSS rules to calculate precise graphical coordinates for the visual representation it will paint on the screen.^[1]

Some engines may begin rendering before all of 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.

Notable engines

Apple created the WebKit engine for its Safari browser by forking the KHTML engine of the KDE project.^[4] All browsers for iOS must use WebKit as their engine.^[5]

Google originally used WebKit for its Chrome browser but eventually forked it to create the Blink engine.^[6] 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 is only used for UWP apps, as the Edge browser was remade with the Blink engine.^[7]

Mozilla develops the Gecko engine for its Firefox browser, the Thunderbird email client, and the SeaMonkey internet suite.^[2]

Timeline

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

References

^ ^a ^b "Behind the scenes of modern web browsers". Tali Garsiel. Retrieved 2018-04-21.
^ ^a ^b "Gecko". Mozilla. Retrieved 2018-04-21.
^ "Introducing Goanna". M.C. Straver. 2015-06-22. Retrieved 2018-04-21.
^ Paul Festa (2003-01-14). "Apple snub stings Mozilla". CNET Networks. Archived from the original on 2012-10-25. Retrieved 2017-02-16.
^ "Open-sourcing Chrome on iOS!". 2017. Retrieved 26 April 2021.
^ Bright, Peter (April 3, 2013). "Google going its own way, forking WebKit rendering engine". Ars Technica. Conde Nast. Retrieved March 9, 2017.
^ Mackie, Kurt (10 December 2018). "Microsoft Edge Browser To Get New Rendering Engine but EdgeHTML Continues". Redmond Mag. Retrieved 21 December 2019.

[behindscene-1] "Behind the scenes of modern web browsers". Tali Garsiel. Retrieved 2018-04-21.

[Gecko-2] "Gecko". Mozilla. Retrieved 2018-04-21.

[Goanna-3] "Introducing Goanna". M.C. Straver. 2015-06-22. Retrieved 2018-04-21.

[4] Paul Festa (2003-01-14). "Apple snub stings Mozilla". CNET Networks. Archived from the original on 2012-10-25. Retrieved 2017-02-16.

[5] "Open-sourcing Chrome on iOS!". 2017. Retrieved 26 April 2021.

[6] Bright, Peter (April 3, 2013). "Google going its own way, forking WebKit rendering engine". Ars Technica. Conde Nast. Retrieved March 9, 2017.

[7] Mackie, Kurt (10 December 2018). "Microsoft Edge Browser To Get New Rendering Engine but EdgeHTML Continues". Redmond Mag. Retrieved 21 December 2019.

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