`<!DOCTYPE ...>`

Usage Recommendation
Use this tag if you want to use an SGML/HTML validator. Otherwise, this tag is of little use.

The <!DOCTYPE ...> declaration (technically it's not a "tag") should be the very first thing in your document... if you choose to use it at all. <!DOCTYPE ...> tells the browser what version of HTML you are writing in. More specifically, <!DOCTYPE ...> declares that this document conforms to a specific version of HTML, and specifies what version that is.

The necessity of <!DOCTYPE ...> is a subject of much debate. The standards published by W³C require the use of <!DOCTYPE ...>. However, much of the HTML being written does not conform strictly the W³C specifications, and so using <!DOCTYPE ...> (which is, after all, a claim that you conform to the standards) would seem unnecessary.

However, <!DOCTYPE ...> has its place on a page, even if you decide to use a little non-standard markup. <!DOCTYPE ...>'s most useful purpose is when used in conjunction with one of the various HTML analyzers. The analyzers look to the <!DOCTYPE ...> to determine what type of HTML you are trying to write. Our recommendation is that you use the appropriate <!DOCTYPE ...>. It won't hurt and allows you to make use of the HTML checkers available.

This is the <!DOCTYPE ...> declaration for HTML version 3.2:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">

For HTML 4.0, the situation is a little more complicated. There are three standard doctypes. The DTD for documents that strictly conform (don't use any deprecated markup) and that aren't frameset documents, use this <!DOCTYPE ...>:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN" 
"http://www.w3.org/TR/REC-html40/strict.dtd">

For a not quite so strict conformance (uses some of the deprecated markup such as <CENTER ...>), use this <!DOCTYPE ...>:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
"http://www.w3.org/TR/REC-html40/loose.dtd">

Finally, for documents which are frameset documents (the "top" document in a framed page), use this <!DOCTYPE ...>:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Frameset//EN"
"http://www.w3.org/TR/REC-html40/frameset.dtd">

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