Web Design in a Nutshell, 3rd Edition

Three Flavors of HTML 4.01 and XHTML 1.0

Although the W3C has ideas on how HTML should work, they are also aware that it is going to be a while before old browsers are phased out and web authors begin to mark up documents properly. For that reason, both the HTML 4.01 and XHTML 1.0 Recommendations encompass three slightly different specification documents: one “Strict,” one “Transitional,” and one just for framed documents. These documents, called Document Type Definitions (or DTDs), define every element, attribute, and entity along with the rules for their use. The XHTML DTDs are written following the rules and conventions of XML (Extensible Markup Language), while the HTML DTDs follow SGML syntax. See Chapter 7 for more on XML. The browser uses the DTD to “decode” the markup and check it for validity.

Strict DTD

This version excludes all deprecated elements and attributes (such as font and align) to reinforce the separation of document structure from presentation. Ideally, documents should be tagged strictly for meaning and structure, leaving all presentation to be handled by style sheets.

Transitional DTD

The Transitional DTD includes all deprecated elements and attributes in order to be backwards compatible with the legacy behavior of most browsers. Deprecated elements and attributes are permitted but discouraged from use. This DTD provides a way to ease web authors out of their current habits and toward abiding by standards . Many web authors today choose to use the Transitional ...