Defines the HTML DTD (Document Type Definition) that the document adhere to.
<!DOCTYPE>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.0//EN">
The HTML document with this tag should compatible with the HTML
3.0 DTD.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Strict//EN">
The HTML document with this tag should adhere to the HTML 4.0
DTD strictly.
The document prologue identifier should be in the first line of any HTML document. Different versions can be used depending on the level of compatibility the author wishes to indicate.