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 Standard Generalized Markup Language (SGML) 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
<!DOCTYPE> to use
if you include nonstandard, albeit popular extensions in the HTML
document—even for the deprecated HTML 3.0 standard, for which a DTD was
HTML developers are increasingly including an appropriate SGML DOCTYPE command as a prefix in their HTML documents. Because of the confusion of versions and standards, if you do choose to include a DOCTYPE in your HTML document, choose the appropriate one to ensure that your document is rendered correctly.
For XHTML authors, we do strongly recommend that you include the proper DOCTYPE statement in your XHTML documents, in conformance ...