W3C decided to reincarnate HTML development.
Phylosophical difference between HTML and XHTML can be demonstraed by the fact that tag <html> is optional in HTML and is not in XHTML. By definition of XML (XHTML):
Definition: A data object is an XML document if it is well-formed, as defined in this specification. In addition, the XML document is valid if it meets certain further constraints.
What means that if UA (user agent, browser) can deal with loading XHTML document only when it receives closing </html> and everything inside is valid. To be short: any partial markup is not XHTML by definition and can be interpretted as such.