Every HTML document should conform to the HTML SGML DTD, the formal Document Type Definition that defines the HTML standard. The DTD defines the tags and syntax that are used to create an HTML document. You can inform the browser which DTD your document complies with by placing a special SGML (Standard Generalized Markup Language) command in the first line of the document:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
This cryptic message indicates that your document is intended to be compliant with the HTML 4.01 final DTD defined by the World Wide Web Consortium (W3C). Other versions of the DTD define more restricted versions of the HTML standard, and not all browsers support all versions of the HTML DTD. In fact, specifying any other doctype may cause the browser to misinterpret your document when displaying it for the user. It’s also unclear what doctype to use when including in the HTML document the various tags that are not standards but are very popular features of a popular browser — the Netscape extensions, for instance, or even the deprecated HTML 3.0 standard, for which a DTD was never released.
Almost no one precedes their HTML documents with the SGML doctype command. Because of the confusion of versions and standards, we don’t recommend that you include the prefix with your HTML documents either.
On the other hand, we do strongly recommend that you include the proper doctype statement in your XHTML documents, in conformance with XML standards. ...