A server normally assigns HTML files a MIME type of text/html, ie. it is served as HTML.

A server normally sends HTML 4.01 files with a MIME type of text/html. HTML is an SGML application.

Things are not so straightforward when dealing with XHTML 1.0, which is XML-based.

Many people prefer to use XHTML because of the advantages XML brings for editing or processing of documents. However, there is still a lack of support for XML files in mainstream browsers, so many XHTML 1.0 files are actually served using the text/html MIME type. In this case, the user agent will treat the file as HTML.

To ensure that the slight differences between XML and HTML do not trip up older user agents, you should always follow the compatibility guidelines in Appendix C of the XHTML specification when serving XHTML as HTML. These compatibility guidelines recommend, amongst other things, that you leave a space before the '/>' at the end of an empty tag (such as img, hr or br), that you always use both id and name attributes for fragment identifiers, etc.