there will always have users who do not upgrade their browser for reason X or Y (most of the time due to computer conf.)
but do not focus on browsers when you create a style... if you use basic elements, all browsers will see it properly. never use InternetExplorer specific codes so you are compliant with the entire
www...
you can read some articles about your situation:
http://www.alistapart.com/articles/minorthreat/