Gecko (layout engine)

Gecko is a layout engine currently developed by Mozilla Corporation, known as the layout engine of the Firefox web browser, Mozilla Application Suite, Nvu, Mozilla Thunderbird and many more. It is designed to support open Internet standards, and is used by applications such as Mozilla Firefox, Camino, Flock, SeaMonkey, K-Meleon, Netscape 9, Lunascape 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.^[1] Development originated with Netscape Communications Corporation, but soon moved to the Mozilla Foundation for the Mozilla application suite, and is now used in many applications developed by Mozilla Foundation and 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, and runs on various operating systems including BSDs, Linux, Mac OS X, Solaris, OS/2, AIX, OpenVMS, and Microsoft Windows. Its development is now overseen by the Mozilla Foundation. Licensed by 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 & Google Chrome) and Presto (used by Opera).

Standards support

(category)
Contents
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Origins and Lineage
Netscape Navigator Mozilla Foundation Mozilla Suite

From the outset, Gecko was designed to support open Internet standards. Some of the standards Gecko supports include:

• HTML4 (partial support for HTML5)
• CSS Level 2.1 (partial support for CSS 3)^[2]
• JavaScript 1.8 (ECMAScript 3, and partial support for ECMAScript 5), implemented in SpiderMonkey
• DOM Level 1 and 2 (partial support for DOM 3)
• XML 1.0
• XHTML 1.0
• XSLT and XPath, implemented in TransforMiiX
• MathML
• XForms (via an official extension)
• RDF

Gecko also partially supports SVG 1.1.^[3][4]

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 marquee element 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.

History

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 NGLayout (next generation layout) due to trademark 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)^[5], eventually the Gecko branding won out.

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^[6]. 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.

Usage

Gecko is primarily used in web browsers, notably Firefox. It is also used in other Mozilla web browser derivatives such as Camino, Flock, SeaMonkey, K-Meleon, Netscape and the version of Internet Explorer that runs under Wine^[7].

Google's picture-organization software Picasa (for Linux) is based on Gecko.^[8].

DevHelp, a GTK+/GNOME browser for API documentation, uses Gecko for rendering docs.^[9]

Other products not included in the table include Swiftfox, Portable Firefox, Fennec, Conkeror, Classilla, HP Secure Web Browser, Oxygen, Minimo, Sylera (for mobile), Thunderbird (email), Sunbird (calendar), Songbird (music player) and Instantbird.

Gecko is also used by Sugar for the OLPC XO-1 computer.^[10] Gecko is used as a complete implementation of the XUL (XML User Interface Language). Gecko currently defines the XUL specification.

The following table compares the different versions of Gecko:

† – Browser also includes WebKit as an alternative rendering engine.
‡ – Browser also includes Trident as an alternative rendering engine.
§ – Alpha and Nightly builds until June 28, 2010 used Firefox version 3.7 and Gecko version 1.9.3.

The following table compares the different versions of Gecko:

Criticism

One of the primary reasons for slower market share adoption is the complexity of the Gecko code which aims to provide much more than just an HTML renderer for web browser.^[11][12][13] The Gecko engine also provides a versatile XML-based user interface rendering framework called XUL that was used extensively in mail, newsgroup and other programs. Another reason for much of the complexity in Gecko is the use of XPCOM a cross-platform component model^[14].

References

External links

• Gecko - Mozilla Developer Center
• Gecko development wiki (wiki.mozilla.org)
• Gecko Layout Engine - Real-time Reflow Videos
• Mozilla Product Strategy Proposal (DRAFT)
• Web Browser Standards Support
• JRex (Java API for Gecko)