Web 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". 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 Chromium 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.

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. All browsers for iOS must use WebKit as their engine.
* Google originally used WebKit for its Chrome browser but eventually forked it to create the Blink engine. 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. EdgeHTML is still used in some UWP apps.
* Mozilla develops the Gecko engine for its Firefox browser and the Thunderbird email client.

Timeline

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

# header
ImageSize = width:750 height:170
PlotArea = left:80 right:5 bottom:20 top:5
AlignBars = justify
DateFormat = dd/mm/yyyy
Period = from:01/01/1997 till:31/05/2022
TimeAxis = orientation:horizontal format:yyyy

Colors =
id:microsoft value:rgb(0.3, 0.5, 1)
id:gecko value:rgb(1, 0, 0)
id:khtml value:rgb(0.3,0.3,0.3)
id:webkit value:rgb(0.3, 0.36, 0.36)
id:blink value:rgb(0.3, 0.39, 0.39)
id:mgray value:rgb(0.5, 0.5, 0.5)
id:lgray value:rgb(0.75, 0.75, 0.75)

ScaleMajor = gridcolor:mgray unit:year increment:1 start:01/01/1997
ScaleMinor = unit:month increment:3 start:01/01/1997
BackgroundColors = bars:lgray

BarData =
bar:Gecko text:Gecko
bar:Trident text:Trident
bar:EdgeHTML text:EdgeHTML
bar:KHTML text:KHTML
bar:WebKit text:WebKit
bar:Blink text:Blink

PlotData=
align:center textcolor:black fontsize:8 mark:(line,black) width:15 shift:(0,-4)

bar:Gecko
from: 01/05/1997 till: 31/05/2022 color:gecko mark:(line,white)

bar:Trident
from: 01/10/1997 till: 15/07/2015 color:microsoft mark:(line,white)

bar:EdgeHTML
from: 12/11/2014 till: 15/12/2018 color:microsoft mark:(line,white)

bar:KHTML
from: 04/11/1998 till: 15/10/2016 color:khtml mark:(line,white)

bar:WebKit
from: 25/06/2001 till: 31/05/2022 color:webkit mark:(line,white)

bar:Blink
from: 03/04/2013 till: 31/05/2022 color:blink mark:(line,white)

See also

* Comparison of browser engines

References

{{Browser engines

Layout engines
Web browsers