It’s imporant that your web site displays well in as many of the mainstream web browsers as possible. If you can make it compatible across platforms too, that’s ideal. Most end users are on a PC so this is probably the most important platform to target. However many people are on a Mac, so it is important to pay attention to this audience as well.
Source: StatCounter Global Stats - Browser Market Share
It will be interesting to see how this progresses over the year as all 5 companies are adding as many features they can to entice users and improve your web surfing experience.
For the most part a good Content Management System such as Joomla will take care of of these things for you.
