DEJA VU! (And I don't mean the
great CSN&Y album!)

The first problem that anyone starting on DHTML encounters (let's say the time is early 1999...) is CROSS-BROWSER COMPATIBILITY. That's what I learned then, and I am glad I learnt it from books like the two I suggested elsewhere in this forum or in JavaRanch in general, before I would be tempted to code JavaScript without having an idea of which browsers would behave in what way.
It turns out that even the same browser version can show different behaviour in different platforms (Netscape in a Mac vs. the same browser in a Sun Workstation, for example).
It's really not worth it for the PUBLIC WEB. In an Intranet is good because someone somewhere
dictates the HW platform and browser to be used.
For the Public Web, server side processing will guarantee that everyone will be serviced the same.
BTW: CSS is also part of DHTML...