6. What is a DTD?
A DTD, or document type definition, specifies the element types and attributes that we can use in our web page. It also defines the rules of how we can use these elements together -- it's the specification for our markup language. The DTD can also declare the character entities we can use; more about those later.
A validator will test a web page for compliance with the DTD specified in the DOCTYPE declaration either explicitly, via the system identifier, or implicitly, using the public identifier. Browsers use non-validating parsers and do not actually read the DTD. They have built-in knowledge about the various element types, and usually a hard-coded list of character entities as well.
For HTML 4.01, which is the latest and greatest version, there are three different DTDs: Strict, Transitional and Frameset.