Three DTDs for XHTML

XHTML comes in three flavors, depending on which DTD you choose:

Strict

This is the W3C’s recommended form of XHTML. This includes all the basic elements and attributes such as p and class. However, it does not include deprecated elements and attributes such as applet and center. It also forbids the use of presentational attributes such as the body element’s bgcolor, vlink, link, and text. These capabilities are provided by CSS instead. Strict XHTML is identified with this DOCTYPE declaration:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
                      "DTD/xhtml1-strict.dtd" >

Example 7-2 uses this DTD.

Transitional

This is a looser form of XHTML for when you can’t easily do without deprecated elements and attributes, such as applet and bgcolor. It is identified with this DOCTYPE declaration:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
                      "DTD/xhtml1-transitional.dtd" >
Frameset

This is the same as the transitional DTD except that it also allows frame-related elements, such as frameset and iframe. It is identified with this DOCTYPE declaration:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN"
                      "DTD/xhtml1-frameset.dtd" >

All three DTDs use the same http://www.w3.org/1999/xhtml namespace. You should choose the strict DTD unless you’ve got a specific reason to use another one.

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