For instance, I've never used JavaScript because I don't want to rely on it. I've found sites that use it when present and "degrade gracefully" in its absence, don't degrade niceley; the wonderful navigation falls completely apart, etc.
I really want to use CSS, it makes life so much easier, but a very large segment of my testing community is on Netscape 4.xx. (But that's changing at the end of the month!!!!
I used to pride myself that all my sites were useable in IE, Netscape, and Lynx. The newest one with the nested table fake of CSS of course is completely unuseable in Lynx.
So I guess my questions for you are:
1. What browsers do you target?
2. What browser(s) do you develop on?
3. What client-side capabilities to you use?
4. What client-side capabilities do you need?
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My answers:
1. Netscape 4 and higher, IE 3(I think) and higer, lynx
2. Netscape 4 (keeps me honest), but changing to Mozilla shortly
3. (X)HTML, subset of CSS
4. HTML