The great thing about CSS and DHTML is that, like HTML, they don't require any special or expensive software. Their code is just text, and you can edit it with a program such as TextEdit (Mac OS) or NotePad (Windows).
The support Web site (webbedenvironments.com/css_dhtml_ajax) includes a list of extremely helpful (and mostly free or cheap) utilities and tools that I recommend to anyone who creates Web sites.
In addition, a couple of programs make life with DHTML and CSS much easier by automating many of the tedious and repetitive tasks associated with Web design. I recommend using Adobe GoLive or Dreamweaver.