The W3C has a number of requirements for documents before they can be called true XHTML documents. Here's the list of requirements that documents must meet:
The document must successfully validate against one of the W3C XHTML DTDs.
The document element must be <html>.
The document element, <html>, must set an XML namespace for the document, using the xmlns attribute. This namespace must be "http://www.w3.org/1999/xhtml.
There must be a <!DOCTYPE> element, and it must appear before the document element.
XHTML is designed to be displayed in today's browsers, and it works well (largely because those browsers ignore elements that they don't understand, such as <?xml?> and <!DOCTYPE>). However, because XHTML is also XML, a number of ...