Forum Moderators: not2easy
In answer to your question, use CSS1 and CSS2. CSS2 is compliant with most UA's. It may display things slightly different (which can be modified with minor hacks), but it works in the following browsers:
IE 4/5/5.5/6
Safari
Opera
Mozilla
Konquerer
and many more.
For things such as Netscape 4, you should only offer a logically structured text-only option of your website. I didn't mention Netscape in the above list, becomes it uses a gecko engine like Mozilla.