When developing a website, you unfortunately have to constantly test it in various browsers and OSs and their many versions because they all seem to behave somewhat differently. One way to easily do this is by installing a VM (virtual machine) on your computer and within one or more VMs, installing various browsers. Oracle / Sun VirtualBox is a fast and easy VM tool that can do this.
When deciding which browsers to test against, you can check your analytics to see the percentage of your site’s visitors by OS and platform and decide whether to support browsers that are used by at least X %.