XHTML 1.0 can be served as HTML or XML. If you serve it as XML, use the MIME type application/xhtml+xml.

It is generally a good idea to use a DOCTYPE declaration at the top of an HTML or XHTML file so that the document is rendered in standards mode by more recent user agents.

The presence of an XML declaration in an XHTML 1.0 file served as HTML will cause your file to be rendered in quirks mode on Internet Explorer (and therefore for a potentially large proportion of your audience).

For more detail on these topics, follow the Related Links in the separate article derived from this section, and check out the pages that they point to.