DTD's
XHTML documents have three parts: the DOCTYPE (which contains the DTD declaration), the head and the body. To create web pages that properly conform to the XHTML 1.0 standard, each page must include a DTD declaration; either strict, transitional, or frameset. Each of the three DTD’s is described (with an example) below:
Strict
You should use the strict DTD when your XHTML pages will be marked up cleanly, free of presentational clutter. You use the strict DTD together with cascading style sheets, because it doesn’t allow attributes like "bgcolor" to be set for the <body> tag, etc.
The strict DTD looks like this:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
Transitional
The transitional DTD should be used when you need to take advantage of the presentational features that are available through HTML. You should also use the transitional DTD when you want to support older browsers that don’t have built-in support for cascading style sheets.
The transitional DTD looks like this:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
Frameset
You should use the frameset DTD when your XHTML page will contain frames. The frameset DTD looks like this:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Frameset//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-frameset.dtd">
It should be fairly obvious which DTD declaration to include in your XHTML file simply by just reading the features of each one. Now that we've got all of the formal details of XHTML out of the way, let's look at some examples of XHTML.
