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LogoutHTML and XHTML are closely related and therefore can be documented together. Both HTML 4.01 and XHTML 1.0 have three sub specifications – strict, loose and frameset. The difference opening declarations for a document distinguishes HTML and XHTML. Other differences are syntactic. HTML allows shortcuts like elements with optional tags, empty elements without end tags. XHTML is very strict about opening and closing tags. XHTML uses built in language defining functionality attribute. All syntax requirements of XML are included in a well formed XHTML document.
Note, though, that these differences apply only when an XHTML document is served as an application of XML; that is, with a MIME type of application/xhtml+xml, application/xml, or text/xml. An XHTML document served with a MIME type of text/html must be parsed and interpreted as HTML, so the HTML rules apply in this case. A style sheet written for an XHTML document being served with a MIME type of text/html may not work as intended if the document is then served with a MIME type of application/xhtml+xml. For more information about MIME types, make sure to read MIME Types.
This can be especially important when you’re serving XHTML documents as text/html. Unless you’re aware of the differences, you may create style sheets that won’t work as intended if the document’s served as real XHTML.
Where the terms “XHTML” and “XHTML document” appear in the remainder of this section, they refer to XHTML markup served with an XML MIME type. XHTML markup served as text/html is an HTML document as far as browsers are concerned.