We know already that the DOCTYPE declaration can be left off a HTML document. All XHTML documents have a mandatory requirement for this declaration, otherwise it will not comply with the rules for well formedness.
<!DOCTYPE HTML
PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
"http://www.w3.org/TR/xhtml1/frameset.dtd"
This is the declaration for a Frameset DTD, and the Transitional DTD is:
<!DOCTYPE HTML
PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/transitional.dtd"
For a Strict DTD declaration it is:
<!DOCTYPE HTML
PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/strict.dtd"
Note that the URL for the DTD has been amended, as it is a different file, and the XHTML version has been inserted.
