By the late 1990's, the W3C (World wide Web Consortium) was interested in standardizing what was called version 4 of HTML. But Explorer and Navigator often played fast and loose with the official version, each in their own way. And the problem of complexity remained. For browsers designed to run on personal computer, this looseness and complexity does not pose a major problem. PCs have plenty of memory and storage space. Everyone could see, however, that a new generation of web-enabled devices like hand-held computers and intelligent cell phones was on the way. The W3C estimated that by 2002 up to 75 percent of computers connected to the Internet would be small web-enabled devices. Like many predictions, their timing was overly optimistic, although clearly the trend is in that direction. Such devices do not have the memory and storage space to handle a full-featured browser displaying full-featured web pages. Nor would most web pages, which are designed to be viewed on PC monitors, display well on the screens of small devices. In addition, there was the problem of HTML not being able to handle nontraditional content such as chemical formulae, musical notation, or mathematical expressions.
To answers these needs, the W3C decided that instead of making an HTML version 5, it would turn to a different language that it had been developing: Extensible Markup Language, or XML. The primary motivation for creating XML was the need to send data over the Internet in a universal, structured format, especially as electronic commerce and data exchange over the Internet began to take off in the 1990's. XML is a set of rules that lets web designers classify their data in a way customized to their needs by creating new types of tags. Thus the name extensible: XML allows designers to extend the language to fit their needs.
The W3C decided that the best approach to solve HTML's problems would be to reorganize and reformulate it from the ground up using XML rules. Because this new version of HTML would not be based on earlier versions of HTML, but on XML, it was named Extensible Hypertext Markup Language, or XHTML. The first version of XHTML was released in january 2000. Navigator 6 and Explorer 6 both support XML for the most part, and thus also support XHTML. In order to ensure that XML is universal, (that it works no matter the device being used), it has has some very strict rules. As an XML-based language, XHTML inherits these rules (and their benefits). For example, unlike HTML, XHTML is a case-sensitive language. The way you type the name of an element matters. The W3C chose to use lowercase letters for XHTML tags. So, even though <BODY> or <body> is acceptable in HTML, only <body> would be correct in XHTML. Two other important rules for using XHTML: all tags must have corresponding ending tags and all tags must be properly nested. Now let us look in more detail at what you will need to create an XHTML document.
NOTE: When using XHTML to create a Web document, your file extension should be html, NOT xhtml. For example, hw2.html is correct, whereas hw2.xhtml is NOT correct.