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The Extensible HyperText Markup Language (XHTML) is a family of current and future document types and modules that reproduce, subset, and extend HTML, reformulated in XML. XHTML family document types are all XML-based, and ultimately are designed to work in conjunction with XML-based user agents.

Unlike HTML, which focuses on describing how data or text is supposed to be displayed, The XML language instead describes what the data is. So, XML is not something that is apparent on a web page, because it does not actually tell your browser how to display the data. As information and data presented on the world wide web became more complex, XML was invented to effectively structure, store, and send this information.

What makes XML truly unique is that there are no predefined tags as is the case with HTML. All of the tags used in HTML have already been defined, such as the paragraph tag, the header tag, and all the various style tags. XML is not defined. You can make your own tags!

XML, forms the basis for a language called XHTML. XHTML is what is known as a meta-language, which is a language for defining a markup language. To put it simply, SGML (Standard Generalized Markup Language) is the basis for HTML. XML is a more refined subset of SGML, and forms the basis for XHTML. On the whole, XHTML is more flexible than HTML.

XHTML was developed for two reasons: (1) to try to create a language that could more effectively convey the meaning of a particular webpage to a computer, and (2) to create a layout for webpages that would be universally understood by browsers running on different platforms or on different types of screens.

This is extremely important, because people are now using a wide variety of gadgets to connect to the internet, as almost every electronic device on the market now comes equipped with email and internet access. Cell phones, palm tops, computers installed in automobiles; they all have built-in web access.

Each of these devices display text and graphics differently, and utilize different platforms and a variety of web browsers. As a result, someone using a cell phone to access a certain web site may not be able to view it properly because the browser running on that cell phone might not be able to display the HTML. The platforms that run on some of these new products and devices are not totally compatible with HTML.

So, it is imperative that most web designers learn to design web pages in XHTML. As almost every electronic device on the market is now equipped with internet access, it is important to use a versatile programming language like XHTML so that your web pages can be viewed and properly formatted across a wide variety of platforms.

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