What MIME type does HTML use?
The HTML serialization must be served using the text/html MIME type. The XHTML serialization must be served using an XML MIME type, such as application/xhtml+xml or application/xml. Unlike the situation as of XHTML1, the HTML specification says that XHTML must no longer be served as text/html. Using the incorrect MIME type (text/html) for XHTML will cause the document to be parsed according to parsing requirements for HTML. In other words, it will be treated as tag soup. Ensuring the use of an XML MIME type is the only way to ensure that browsers handle the document as XML.
