Gecko is the open source, free software web browser layout engine used in all Mozilla-branded software and its derivatives, including later Netscape releases. Written in C++, Gecko is designed to support open Internet standards. Originally created by Netscape Communications Corporation, its development is now overseen by the Mozilla Foundation.

Gecko offers a rich programming API that makes it suitable for a wide variety of roles in Internet enabled applications, such as web browsers, content presentation and client/server . Primarily it is used by Mozilla browser derivatives, such as Mozilla Firefox, K-Meleon and Netscape, to render websites and the browser user interface (written in XUL), but it is used elsewhere as well. Gecko is cross-platform and works on a number of different operating systems, including Microsoft Windows, BSD s, GNU/Linux and Mac OS X.

Gecko is generally considered to be the second most-popular layout engine on the Web, after Trident (used by Internet Explorer for Windows since version 4), and followed by KHTML (used by Konqueror), WebCore (used by Safari), Presto (used by Opera) and Tasman (used by Internet Explorer for Mac).

Gecko is a layout engine currently developed by Mozilla Corporation, known as the layout engine of the Firefox web browser, Mozilla Application Suite, Nvu, Thunderbird and of many more. It is designed to support open Internet standards, it is used by applications such as Mozilla Firefox 3, Camino, Flock, SeaMonkey, K-Meleon, and Netscape 9, to display web pages and, in some cases, an application's user interface itself (by rendering XUL). Gecko offers a rich programming API that makes it suitable for a wide variety of roles in Internet-enabled applications, such as web browsers, content presentation, and client/server. Development originated with Netscape Communications Corporation, soon switching to the Mozilla Foundation for the Mozilla application suite, and now used in many applications developed by Mozilla Foundation and or the Mozilla Corporation, as well as many other open source software projects. It was also used in later Netscape Navigator releases.

Gecko is written in C++ and is cross-platform, running on operating systems including Microsoft Windows, BSDs, Linux, Mac OS X, Solaris, OS/2, AIX and OpenVMS. Its development is now overseen by the Mozilla Foundation. Licensed a tri-license of the Mozilla Public License (MPL), GNU General Public License (GPL) and GNU Lesser General Public License (LGPL), Gecko is free and open source software.

Gecko is the second most-popular layout engine on the World Wide Web, after Trident (used by Internet Explorer for Windows since version 4), and followed by WebKit (used by Safari) and PrestoOpera).
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From the outset, Gecko was designed to support open Internet standards. Some of the standards Gecko supports include:

  • HTML 4.0
  • CSS Level 1 (partial support for CSS 2 and 3, see Mozilla CSS support chart for details)
  • JavaScript 1.8 (ECMAScript)
  • DOM Level 1 and 2 (partial support for DOM 3)
  • XML 1.0
  • XHTML 1.1
  • XSLT and XPath
  • MathML
  • XForms (via an official extension)
  • RDF

Gecko also partially supports SVG 1.1.

In order to support web pages designed for legacy versions of Netscape and Internet Explorer, Gecko supports DOCTYPE switching. Documents with a modern DOCTYPE are rendered in standards compliance mode, which follows the W3C standards strictly. Documents that have no DOCTYPE or an older DOCTYPE are rendered in quirks mode, which emulates some of the non-standard oddities of Netscape Communicator 4.x, however, some of the 4.x features (such as layers) are not supported.

Gecko also has limited support for some non-standard Internet Explorer features, such as the tag and the document.all property (though pages explicitly testing for document.all will be told it is not supported). While this increases compatibility with many documents designed only for Internet Explorer, some purists argue that it harms the cause of standards evangelism.

Development of the layout engine now known as Gecko began at Netscape in 1997, following the company's purchase of DigitalStyle. The existing Netscape rendering engine, originally written for Netscape Navigator 1.0 and upgraded through the years, was widely considered to be inferior to the one used in Microsoft Internet Explorer. It was slow, did not comply well with W3C standards, had limited support for dynamic HTML and lacked features such as incremental reflow (when the layout engine rearranges elements on the screen as new data is downloaded and added to the page). The new layout engine was developed in parallel with the old, with the intention being to integrate it into Netscape Communicator when it was mature and stable. At least one more major revision of Netscape was expected to be released with the old layout engine before the switch.

After the launch of the Mozilla project in early 1998, the new layout engine code was released under an open-source license. Originally unveiled as Raptor, the name had to be changed to NGLayouttrademark problems. Netscape later rebranded NGLayout as Gecko. While Mozilla Organization (the forerunner of the Mozilla Foundation) initially continued to use the NGLayout name (Gecko was a Netscape trademark), eventually the Gecko branding won out. For a time, Gecko was used to refer to both the old NGLayout layout engine and XPFE (cross-platform front-end)—the new XML-based Mozilla user interface, rendered by NGLayout—but it is now used solely to refer to the layout engine. (next generation layout) due to

In October 1998, Netscape announced that its next browser would use Gecko (which was still called NGLayout at the time) rather than the old layout engine, requiring large parts of the application to be rewritten. While this decision was popular with web standards advocates, it was largely unpopular with Netscape developers, who were unhappy with the six months given for the rewrite. It also meant that most of the work done for Netscape Communicator 5.0 (including development on the Mariner improvements to the old layout engine) had to be abandoned. Netscape 6, the first Netscape release to incorporate Gecko, was released in November 2000 (the name Netscape 5 was never used).

As Gecko development continued, other applications and embedders began to make use of it. America Online, by this time Netscape's parent company, eventually adopted it for use in CompuServe 7.0 and AOL for Mac OS X (these products had previously embedded Internet Explorer). However, with the exception of a few betas, Gecko was never used in the main Microsoft Windows AOL client.

On July 15, 2003, AOL laid off the remaining Gecko developers and the Mozilla Foundation (formed on the same day) became the main steward of Gecko development. Today, Gecko is developed by employees of the Mozilla Corporation, employees of companies that contribute to the Mozilla project, and volunteers.