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

Besides "browser engine", two other related terms are commonly used: "layout engine" and "rendering engine".[1][2][3] In theory, layout and rendering (or "painting") could be handled by different engines. In practice, however, these components are tightly coupled and rarely encountered on their own outside of the browser engine.[1][4]

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.[1][4]

Every major browser supports JavaScript to provide a wide range of dynamic behavior for web pages. However, JavaScript is implemented as a separate JavaScript engine, which has enabled its usage elsewhere. In a browser, the two engines are coordinated via the DOM and Web IDL bindings.[4]

Browser engines are also used in non-browser applications. An email client needs one to display HTML email. Beginning in the 2010s, many apps have been created with the frameworks based on Google's Chromium project; each of these standalone apps functions much like a web app. (Two examples are Spotify and Slack.)[5][6]

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 for how the page should be presented. For example, some rules specify typography details, such as font, color, and text size, while others determine the placement of images. The engine combines all relevant CSS rules to calculate precise graphical coordinates for the visual representation it will paint on the screen.[1][4]

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.[7]

Notable engines

Further information: Comparison of browser engines


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

See also


  1. ^ a b c d "Behind the scenes of modern web browsers". Tali Garsiel. Retrieved 21 April 2018.
  2. ^ a b "Gecko". Mozilla. Archived from the original on 4 June 2014. Retrieved 21 April 2018.
  3. ^ "Introducing Goanna". M.C. Straver. 22 June 2015. Retrieved 21 April 2018.
  4. ^ a b c d "How Blink Works". Google. Retrieved 12 March 2024.
  5. ^ "Open Source - Spotify". Retrieved 1 December 2023. Here are the sources to the great Chromium Embedded Framework that is used by the Spotify Desktop client.
  6. ^ Betts, Anaïs (25 October 2016). "Building Hybrid Applications with Electron". Slack Engineering. Slack. Retrieved 1 December 2023.
  7. ^ Boudreaux, Ryan (18 October 2013). "How to prevent Flash of Unstyled Content on your websites". TechRepublic. Archived from the original on 5 March 2021. Retrieved 9 October 2021.
  8. ^ Paul Festa (14 January 2003). "Apple snub stings Mozilla". CNET Networks. Archived from the original on 25 October 2012. Retrieved 16 February 2017.
  9. ^ "Open-sourcing Chrome on iOS!". 2017. Retrieved 26 April 2021. Due to constraints of the iOS platform, all browsers must be built on top of the WebKit rendering engine.
  10. ^ Bright, Peter (3 April 2013). "Google going its way, forking WebKit rendering engine". Ars Technica. Conde Nast. Retrieved 9 March 2017.
  11. ^ Mackie, Kurt (10 December 2018). "Microsoft Edge Browser To Get New Rendering Engine but EdgeHTML Continues". Redmond Mag. Retrieved 21 December 2019.
  12. ^ Mendelevich, Alan (14 May 2021). "You Think You Can Forget About the "Legacy" Microsoft Edge? Not So Fast!".