Each HTML document must begin with a document type declaration that declares which version of HTML the document adheres to. HTML 4 comes in three flavors, each with a different DOCTYPE:
HTML 4 Strict
HTML 4 Strict is a trimmed down version of HTML 4 that emphasizes structure over presentation. Deprecated elements and attributes (including most presentational attributes), frames, and link targets are not allowed in HTML 4 Strict. By writing to HTML 4 Strict, authors can achieve accessible, structurally rich documents that easily adapt to style sheets and different browsing situations. However, HTML 4 Strict documents may look bland on very old browsers that lack support for style sheets.
The document type declaration for HTML 4.01 Strict is
"http://www.w3.org/TR/html4/strict.dtd">
Newer browsers such as Internet Explorer 5 for Mac, Netscape 6, and Mozilla use a standards-compliant rendering for HTML 4 Strict documents. These browsers use a "quirks" mode for most other document types to emulate rendering bugs in older browsers.
HTML 4 Transitional
HTML 4 Transitional includes all elements and attributes of HTML 4 Strict but adds presentational attributes, deprecated elements, and link targets.
The document type declaration for HTML 4.01 Transitional is
"http://www.w3.org/TR/html4/loose.dtd">
Newer browsers such as Internet Explorer 5 for Mac, Netscape 6, and Mozilla use a standards-compliant rendering for HTML 4.01 Transitional documents that include the URI of the DTD in the DOCTYPE. These browsers use a "quirks" mode to emulate rendering bugs in older browsers if the URI is omitted:
HTML 4 Frameset
HTML 4 Frameset is a variant of HTML 4 Transitional for documents that use frames. The FRAMESET element replaces the BODY in a Frameset document.
The document type declaration for HTML 4.01 Frameset is
"http://www.w3.org/TR/html4/frameset.dtd">
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