Verify that a document is correct XHTML
From the XHTML 1.0 spec:
It must validate against one of the three DTDs found in Appendix A.
The root element of the document must be
<html>.
The root element of the document must designate the XHTML namespace using the
xmlnsattribute [XMLNAMES]. The namespace for XHTML is defined to behttp://www.w3.org/1999/xhtml.
There must be a DOCTYPE declaration in the document prior to the root element. The public identifier included in the DOCTYPE declaration must reference one of the three DTDs found in Appendix A using the respective Formal Public Identifier. The system identifier may be changed to reflect local system conventions.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "DTD/xhtml1-strict.dtd"> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "DTD/xhtml1-transitional.dtd"> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "DTD/xhtml1-frameset.dtd">