XML Unleashed

XHTML and Document Validity

In order for you to validate XHTML documents, there must be a DTD that carefully describes the structure of XHTML. Instead of developing a single DTD for XHTML document validation, the W3C went a step further and developed three DTDs, which are all included in the XHTML specification. These DTDs are approximations of the equivalent SGML DTDs and provide varying levels of detail for XHTML, which results in three different classifications of XHTML documents. The idea is that you can use a more minimal XHTML DTD if you don't need to use certain XHTML language features; if you do, you can use a more thorough DTD. The three XHTML DTDs are classified as follows, in order of increasing functionality:

• Strict—No HTML presentation ...