XHTML: The X stands for "Extensible", and this kind of HTML works hand in hand with XML (eXtensible Markup Language) to enable people to create more customized tags with specific behaviors attached to each one.
Now, with XHTML and cascading style sheets, you can use a number of different methods to program a guestbook once and have the code do all the work.
More and more web designers are moving away from HTML and into the more intuitive and flexible world of XML, XHTML, CSS, and other web coding tools.
XHTML-CSS - XHTML stands for "extensible" HTML, a form of HTML that allows for more functionality and a greater interoperability across platforms.
Part of XHTML is the creation of "classes" along with the usual tags, making the code easier to write and easier to read for designers.
But now, a designer can use flash in conjunction with standard HTML, CSS, XHTML and DHTML to fully animate a website.
You can create CSS style sheets and even program with the XHTML code, if you desire.