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Building according to web standards: Extensible Hyper Text Markup Language (XHTML) 1.0
Although HTML and XHTML have great similarities, use of XHTML can be complicating. Web developers who consider using XHTML should read this chapter through thoroughly.
HTML is a markup language following the rules of a much more extensive and older language, SGML (Standard Generalized Markup Language). XML (Extensible Markup Language) is likewise derived from SGML, but its transparency and extensibility makes implementation in various applications (not just web browsers) much easier. XML is a markup language for describing markup languages; XML enables developers to write new markup languages. XHTML is such a ‘new’ markup language.
Use HTML 4.01 or XHTML 1.0 according to the W3C specifications for the markup of websites.
Guideline R-pd.2.1
What is XHTML?
XHTML is a reformulation of HTML according to the rules of XML. 
XHTML for websites
It is recommended that XHTML be used only if the web developer is aware of its advantages and disadvantages. There is no preference for either HTML or XHTML. However, use of XHTML 1.1 is not recommended for the time being, owing to the strict characteristics of this standard and poor web browser support.
Links and references
- The XHTML specification
- Rated XHTML,
Peter-Paul Koch - Better Living Through XHTML,
Jeffrey Zeldman
