A layout engine, or rendering engine, is software that takes marked up content (such as HTML, XML, image files, etc.) and formatting information (such as CSS, XSL, etc.) and displays the formatted content on the screen. It "paints" on the content area of a window, which is displayed on a monitor or a printer. A layout engine is typically used for web browsers, e-mail clients, or other applications that require the displaying (and editing) of web contents.
The term "layout engine" only reached popular usage when these became easily separable from the browser. For example, Gecko, the Mozilla project's open-source layout engine, is used by a variety of products derived from the Mozilla code base, including the Firefox web browser, the Thunderbird E-mail client, and Seamonkey application suite. Trident, the layout engine from Internet Explorer, is used by many applications on the Microsoft Windows platform to render HTML, as in Outlook Express, some versions of Microsoft Outlook, and the mini-browsers in Winamp and RealPlayer.
Similarly, Opera Software's proprietary Presto engine is licensed to a number of other software vendors, as well as being used in Opera's own Opera web browser, and KDE's open-source KHTML engine is used both in KDE's own Konqueror web browser as well as being used, in an adapted form, as the basis for the rendering engine in Apple's Safari web browser.
The term rendering engine can also refer to text rendering engines like Pango or Uniscribe which make multilingual texts present in proper shape, taking into account bidirectional text, combinations of "basic characters" with accents, and other intricacies of multilingual text.
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