HTML4.01中的DTD

Question

The purpose of a DTD (Document Type Definition) is to define the legal building blocks of an XML document. So Why it is used in HTML 4.01 as Doctype ?

And what about HTML5 If it is not using dtd then how can we validate HTML5 documents. Do we have to write our custom DTD here ?

Answer 1

The purpose of a DTD (Document Type Definition) is to define the legal building blocks of an XML document.

SGML documents too.

So Why it is used in HTML 4.01 as Doctype ?

Because HTML 4 is an SGML application and makes use of the pre-existing work done to make SGML useful.

And what about HTML5

The authors of HTML 5 decided that since browser vendors all implemented tag soup parsers instead of SGML parsers, HTML 5 wouldn't be an SGML application.

If it is not using dtd then how can we validate HTML5 documents.

To validate an HTML 5 document you need to use an HTML 5 validator instead of a general SGML validator.