Each HTML document must begin with a document type declaration that declares which version of HTML the document adheres to. HTML 4.0 comes in three flavors, each with a different DOCTYPE:
HTML 4.0 Strict is a trimmed down version of HTML 4.0 that emphasizes structure over presentation. Deprecated elements and attributes (including most presentational attributes), frames, and link targets are not allowed in HTML 4.0 Strict. By writing to HTML 4.0 Strict authors can achieve accessible, structurally rich documents that easily adapt to style sheets and different browsing situations. However, since many browsers lack full support for style sheets, HTML 4.0 Strict documents may look bland on common visual browsers such as Netscape Navigator 3.x.
The document type declaration for HTML 4.0 Strict is
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN" "http://www.w3.org/TR/REC-html40/strict.dtd">
HTML 4.0 Transitional includes all elements and attributes of HTML 4.0 Strict but adds presentational attributes, deprecated elements, and link targets. HTML 4.0 Transitional recognizes the relatively poor browser support for style sheets, allowing many HTML presentation features to be used as a transition towards HTML 4.0 Strict.
The document type declaration for HTML 4.0 Transitional is
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd">
HTML 4.0 Frameset is a variant of HTML 4.0 Transitional for documents that use frames. The FRAMESET element replaces the BODY in a Frameset document.
The document type declaration for HTML 4.0 Frameset is
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Frameset//EN" "http://www.w3.org/TR/REC-html40/frameset.dtd">