CSS, javascript, ajax and different browsers

In dealing with customer web pages (order entry) how are others handling browser differences?
Do you dumb the features down to a level most browser accept or code sections for different browsers? This is search & data entry & not so much point & click, & project on a skinny budget.

Depending upon whose stats (this is w3schools),
Internet Exp8 16.2% IE7 7.8% IE6 6.7% Total 30.7
Firefox 4.0 0.6% 3.6 35.2% 3.5 6.1% Total 45.8
Chrome 6.0 1.4% 5.0 15.1% 4.0 0.3% Total 17.0
Safari S5 2.6% ... 3.5

We want to set a minimum standard for browsers.
All of this will be in rpgle cgi.
Jim Franz