XHTML file stores data using standard markup language based on XML and HTML languages. XHTML is not a separate language but an XML extension to HTML 4.01 standard.

XHTML documents need to follow more restrictive rules regarding syntax because XHTML specification introduced more strict error handling when compared to HTML. An error in XHTML document will prevent web browser from displaying the content. One of the strongest points of XHTML is that it allows for mixing in other XML-based content (SVG, for example) in XHTML documents. A valid XHTML document must contain an XML version in the XML declaration at the beginning of the file.