XML (Extensible Markup Language) is a set of rules for encoding documents electronically. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis open standards.
XML's design goals emphasise simplicity, generality, and usability over the Internet. It is a textual data format, with strong support via Unicode for the languages of the world. Although XML's design focuses on documents, it is widely used for the representation of arbitrary data structures, for example in web services.
There are many programming interfaces that software developers may use to access XML data, and several schema systems designed to aid in the definition of XML-based languages.
As of 2009, hundreds of XML-based languages have been developed, including RSS, Atom, SOAP, and XHTML. XML-based formats have become the default for most office-productivity tools, including Microsoft Office (Office Open XML), OpenOffice.org (OpenDocument), and Apple's iWork.

Stands for "Extensible Hypertext Markup Language." Yes, apparently "Extensible" starts with an "X." XHTML is a spinoff of the hypertext markup language (HTML) used for creating Web pages. It is based on the HTML 4.0 syntax, but has been modified to follow the guidelines of XML, the Extensible Markup Language. Therefore, XHTML 1.0 is sometimes referred to as HTML 5.0.
Because XHTML is "extensible," Web developers can create their own objects and tags for each Web page they build. This gives the developers more control over the appearance and organisation of their Web pages. The only requirement is that the custom tags and attributes are defined in a document type definition (DTD), that is referenced by the XHTML page.
XHTML pages must also conform to a more strict syntax than regular HTML pages. While Web browsers are rather lenient and forgiving of HTML syntax, XHTML pages must have perfect syntax. This means no missing quotes or incorrect capitalisation in the markup language. While the strict syntax requires more meticulous Web page creation, it also ensures Web pages will appear more uniform across different browser platforms.


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