If you know HTML, you'll have no problem writing XHTML, because for all practical purposes, it is the same thing. If you'd like to learn more about the subtle differences between HTML and XHTML, please try this article.
XHTML is an acronym for "Extensible HyperText Markup Language", a reformulation of HTML 4.0 as an XML 1.0 application. It is a family of XML markup languages that mirror or extend versions of the widely used Hypertext Markup Language (HTML), the language that web pages are written in.
While HTML (prior to HTML 5) was defined as an application of Standard Generalized Markup Language (SGML), a very flexible markup language framework, XHTML is an application of XML, a more restrictive subset of SGML. Because XHTML documents need to be well-formed, they can be processed using standard XML tools; unlike HTML, which requires a relatively complex, lenient, and generally custom parser.
