The W3Cs XHTML 1.0 Specification provides three document type definitions that can be used to create XHTML webpages:
Strict DTD - This is the standard that should be used for all XHTML pages that only contain fully valid XHTML markup code.
Transitional DTD -This is the standard that can be used where an XHTML document also contains older HTML markup code that is not valid in the strict DTD.
Frameset DTD - The standard that can be used where frames are used to display XHTML pages in a multi-document web page. Small mobile devices do not usually have this capability so it is best to avoid using frames to display XHTML webpages.