HTML Validation: Choosing a DOCTYPE

Choosing a DOCTYPE

According to HTML standards, each HTML document requires a document type declaration. The "DOCTYPE" begins the HTML document and tells a validator which version of HTML to use in checking the document's syntax.

If standard HTML does not meet your needs but you still wish to gain the benefits of HTML validation, see the section on using a custom DTD.

The following DOCTYPEs are commonly used:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
 "http://www.w3.org/TR/html4/strict.dtd">

This declares the document to be HTML 4.01 Strict. HTML 4.01 Strict is a trimmed down version of HTML 4.01 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, since many browsers lack full support for style sheets, HTML 4 Strict documents may look bland on older visual browsers such as Netscape Navigator 3.x.

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.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
 "http://www.w3.org/TR/html4/loose.dtd">

This declares the document to be HTML 4.01 Transitional. HTML 4 Transitional includes all elements and attributes of HTML 4 Strict but adds presentational attributes, deprecated elements, and link targets. HTML 4 Transitional recognizes the relatively poor browser support for style sheets, allowing many HTML presentation features to be used as a transition towards HTML 4 Strict.

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:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN"
 "http://www.w3.org/TR/html4/frameset.dtd">

This declares the document to be HTML 4.01 Frameset. HTML 4 Frameset is a variant of HTML 4 Transitional for documents that use frames.

<!DOCTYPE html 
     PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
     "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

This declares the document to be XHTML 1.0 Strict. XHTML 1.0 Strict is an XML version of HTML 4 Strict.

<!DOCTYPE html 
     PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
     "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

This declares the document to be XHTML 1.0 Transitional. XHTML 1.0 Transitional is an XML version of HTML 4 Transitional.

<!DOCTYPE html 
     PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
     "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">

This declares the document to be XHTML 1.0 Frameset. XHTML 1.0 Frameset is an XML version of HTML 4 Frameset.

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

This declares the document to be HTML 3.2. HTML 3.2 is well supported by most browsers in use. However, HTML 3.2 has limited support for style sheets and no support for HTML 4 features such as frames and internationalization.

<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML//EN">

This declares the document to be HTML 2.0. HTML 2.0 is widely supported by browsers but lacks support for tables, frames, and internationalization, as well as many commonly used presentational elements and attributes.