document using text/html . In other words, requestors that advertise they support XHTML family documents will receive the document in the XHTML media type, and all other requestors that (at least claim to) support HTML or "everything" will receive the document using the HTML media type. Dealing with user agents that satisfy none of these criteria is outside the scope of this document. When an XHTML document does NOT adhere to the guidelines, it should only be delivered as media type application/xhtml+xml . The 'application/xhtml+xml' media type [ RFC3236 ] is the primary media type for XHTML Family documents. 'application/xhtml+xml' should be used for serving XHTML documents to XHTML user agents (agents that explicitly indicate they support this media type). This media type must be used when writing documents using XHTML Family document types that add elements and attributes from foreign namespaces, such as XHTML+MathML [ XHTML+MathML ]. 3.2. 'text/html' The 'text/html' media type [ RFC2854 ] is primarily for HTML, not for XHTML. In general, this media type is NOT suitable for XHTML except when the XHTML is conforms to the guidelines in Appendix A . In particular, 'text/html' is NOT suitable for XHTML Family document types that add elements and attributes from foreign namespaces, such as XHTML+MathML [ XHTML+MathML ]. XHTML documents served as 'text/html' will not be processed as XML [ XML10 ], e.g., well-formedness errors may not be detected by user agents. Also be aware that HTML rules will be applied for DOM and style sheets