1. XHTML must be served with an XML MIME type, such as application/xml or application/xhtml+xml.

2. HTML must be served as text/html.

It is the MIME type that determines what type of document you are using. If you use attempt to send XHTML as text/html, you are actually just using HTML, possibly with syntax errors.

Technically, according to the spec, XHTML 1.0 is allowed to be served as text/html. But, due to the above reason, such a document is considered to be an HTML document, not an XHTML document.

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