Document Type Definition
HTML is responsible for telling a Web browser how text and other objects in a Web Page should appear. As per HTML standards, each HTML document requires a document type declaration (DTD). DOCTYPE is a declaration and always appears at the very top of your documents.
The above code declares the document to be HTML 4.01 Strict
The DOCTYPE statement indicates the particular version of HTML being used in the document. It is used to check the document for validity. DOCTYPE declaration is not an HTML tag. It is only a declaration and always appears at the first line of your HTML documents.
Commonly used DTDsHTML 5
HTML5 is not based on SGML, uses the DOCTYPE only for mode selection.HTML 4.01 Strict
It does not allow Framesets, presentational or deprecated elements.HTML 4.01 Transitional
Transitional DTD allows some older PUBLIC and attributes that have been deprecated, Framesets are not allowed.HTML 4.01 Frameset
Framesets allowed, and all elements and attributes are equal to HTML 4.01 Transitional