When an XHTML document is created, the DTD to which it conforms is
declared at the top of the document. Each DTD may be recognized by a
unique label called a Formal Public Identifier, or FPI. The literal, or quoted, text following the word
PUBLIC is an FPI referring to the W3C's XHTML 1.0 DTD.
Currently, there are three XHTML document types:
Use this with CSS when you
want really clean markup, free of presentational clutter. Several tags
have been removed from the language (like
<center>), and even some attributes of other tags have been removed too (like the
align attribute of the H1 tag).
Use this when you need to take advantage of HTML's presentation features; many of your readers don't have the latest browsers that understand CSS. The transitional DTD supports most of the standard HTML 4 tags and attributes.
This enables you to use HTML frames to partition the browser window into two or more frames. This DTD holds the frameset definitions.