Section 9.2. Document Type Declaration | Web Design in a Nutshell: A Desktop Quick Reference (In a Nutshell (OReilly))

9.2. Document Type Declaration

To be valid, an (X)HTML document must begin with a document type declaration that identifies which version of HTML or XHTML is used in the document. This is done using a DOCTYPE declaration that names the document type definition (DTD) for the document. A DTD is a text document that lists all the elements, attributes, and rules of use for a particular markup language. See Chapter 7 for more information on DTDs.

The inclusion of a document type declaration has always been a requirement of valid HTML documents. With no DOCTYPE declaration, there is no set of rules to validate against. In the years of fast and loose HTML authoring, the DOCTYPE declaration was commonly omitted. However, now that standards compliance is a priority in the web development community, and because there are so many DTDs to choose from, authors are strongly urged to include the DTD declaration and validate their documents. The DOCTYPE declaration (or its omission) also triggers different browser behaviors, as discussed in the upcoming "DOCTYPE Switching" section.

9.2.1. DTD Options

HTML 4.01 and XHTML 1.0 offer three DTD versions:

Strict
Transitional
Frameset

XHTML 1.1 has only one DTD. The DTD documents live on the W3C server at a stable URL.

The <!DOCTYPE> (document type) declaration contains two methods for pointing to DTD information: one is a publicly recognized document identifier; the other is a specific URL in case the browsing device does not recognize the public identifier. Descriptions and specific markup for each HTML and XHTML version are listed here.

HTML 4.01 Strict

The Strict DTD omits all deprecated elements and attributes. If you are authoring according to the strict DTD, use this document type definition:

     <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"      "http://www.w3.org/TR/HTML4.01/strict.dtd">

HTML 4.01 Transitional

The Transitional DTD includes everything from the Strict DTD, plus all deprecated elements and attributes. If your document includes some deprecated elements or attributes, point to the Transitional DTD using this DOCTYPE declaration:

     <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"      "http://www.w3.org/TR/HTML4.01/loose.dtd">

HTML 4.01 Frameset

If your document contains framesthat is, it uses frameset instead of body for its contentthen identify the Frameset DTD. The Frameset DTD is the same as the Transitional version (it includes deprecated yet supported elements and attributes), with the addition of frame-specific elements. The content-containing HTML documents that are displayed within the frames do not need to use the Frameset DTD.

     <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Frameset//EN"      "http://www.w3.org/TR/HTML4.01/frameset.dtd">

XHTML 1.0 Strict

The same as HTML 4.01 Strict, but reformulated according to the syntax rules of XML.

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

XHTML 1.0 Transitional

The same as HTML 4.01 Transitional, but reformulated according to the syntax rules of XML.

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

XHTML 1.0 Frameset

The same as HTML 4.01 Frameset, but reformulated according to the syntax rules of XML.

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

XHTML 1.1

There is only one DTD for XHTML 1.1. It omits every deprecated element and attribute. It differs from XHTML 1.0 Strict in these ways:

The lang attribute has been replaced with the xml:lang attribute.
The name attribute for the a and map elements has been replaced with id.
A ruby collection of elements has been added. The W3C defines ruby as "short runs of text alongside the base text, typically used in East Asian documents to indicate pronunciation or to provide a short annotation."
```
     <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"      "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd"> 
```

The W3C makes these document type declarations and more available for your copy-and-paste convenience at www.w3.org/QA/2002/04/valid-dtd-list.html.

9.2.2. DOCTYPE Switching

Years of lax authoring practices and techniques for dealing with inconsistent browser behaviors resulted in millions of web pages built in a way that worked, but were far from valid against the current standards. Browser developers were faced with a difficult dilemma: get rigorous about standards conformance and break nearly every web site in existence, or maintain the status quo.

When building Internet Explorer 5 for the Macintosh, development lead Tantek Çelik invented and coded a stop-gap solution that served two communities of authors: those writing standards-compliant documents and those who were authoring web documents based on familiar browser rendering behaviors.

The method now known as DOCTYPE switching uses the inclusion and content of a DOCTYPE declaration to toggle the rendering mode in certain browsers. If a modern DOCTYPE declaration is detected, it indicates that the author is standards-aware, and the browser switches into a standards-compliant rendering mode (Standards mode). If no (or if an older) declaration is detected, the browser reverts to Quirks mode. Quirks mode mimics the rendering behavior of old browsers, allowing for nonstandard code, hacks, and workarounds that are common in legacy web authoring practices. There is a third mode that some browsers implement known as Almost Standards mode that is different from true Standards mode in that it implements vertical sizing of table cells traditionally and not according to the CSS 2 specification.

9.2.2.1. Browser support

You can use the DOCTYPE declaration to switch rendering modes in the following browsers:

Internet Explorer 6 and 7 (Windows)
Internet Explorer 5 (Mac)
Netscape 6 and higher
Opera 7 and higher
Mozilla (and Mozilla-based browsers like Firefox)
Safari
Konqueror 3.2 and higher

9.2.2.2. Making the switch

Although all of the browsers listed above do some sort of switching, the requirements for switching them into Standards or Almost Standards mode varies somewhat by browser and is influenced by the DTD version and the presence of the complete URL for that DTD. For XHTML documents, the presence of the XML declaration will cause Internet Explorer 6 for Windows and Konqueror to switch into Quirks mode even if the proper DOCTYPE declaration has been provided.

Figuring out which DOCTYPE triggers which mode in every browser can get pretty confusing. For a thorough comparison of browsers' responses to every possible (X)HTML DTD and URL combination, see the chart created and maintained by Henri Sivonen at http://hsivonen.iki.fi/doctype/.

To summarize here, these are your best bets for triggering Standards or Almost Standards mode in the most browsers that do DOCTYPE switching :

XHTML 1.0 Strict or Transitional or XHTML 1.1, with a complete URL (including http://) and without the XML declaration. If the URL is omitted or incomplete, some browsers revert to Quirks mode. Including the XML declaration causes Internet Explorer 6 to revert to Quirks mode, however, this has been corrected in IE 7.
HTML 4.0 or 4.01 Strict DTD, with or without the URL. (Omitting the URL triggers Quirks mode in IE 5/Mac with the 4.01 Strict DTD only.)
HTML 4.0 or 4.01 Transitional DTD, with the URL http://www.w3.org/TR/html4/loose.dtd (for all browsers but Konquerer 3.2). Including http://www.w3.org/TR/1999/REC-html401-19991224/loose.dtd TRiggers Quirks mode in Netscape 6 and Konqueror.