The reformulation of HTML 4 with XML. XML is a structured set of rules used to define data to be shared on the Web. It is extensible in that anyone can make up a particular set of markups for any purpose and as long as everyone uses the same markups, it will be adapted. HTML has a fixed set of elements that cannot be varied.
With XHTML, new elements could be developed and added to the existing ones. Elements from XML and HTML 4 could be combined, creating new ways to embed content and programming into the Web page. It provides the means for Web authors to enter structured data while maintaining operability with users who support HTML 4. XHTML looks a lot like HTML, so if you are familiar with HTML, it will be easy to learn.