If you’re still using HTML 4 (or an older version of HTML), you should upgrade your code to XHTML. I recommend upgrading to XHTML 1.0 Transitional — this version of XHTML is more forgiving than the Strict version, and if you’re used to HTML 4 the learning curve won't be so steep.

Why do you need to do this? Because of the wide variety of handheld devices and mobile browsers that are available, your code should comply with the current standards that are being promoted by the World Wide Web Consortium (W3C). Also, mobile device browsers support Extensible HTML (XHTML) rather than HTML. If you're using HTML 4 or an even older version of HTML, your code may display poorly, if it displays at all.

Drupal users: go ahead and skip to the next step. The Drupal development community has made it easy to create XHTML-based websites right out of the box!

Dreamweaver users: Dreamweaver has a built-in code conversion feature. Open the document that you want to upgrade, and use File > Convert > XHTML to upgrade the code.

If you code by hand:

• There’s a quick guide to learning what's different about XHTML in Chapter 16 of HTML and XHTML: The Definitive Guide, 6th Edition (link opens in a new window).
• Take a look at You already write for cell phones, a brief introduction to XHTML Basic by Jennifer Kyrnin at About.com (opens in a new window).