As you design web pages, you need to be aware that not everyone will see your pages in the way that you see them on your system. There are different browsers and even with the same brand of browser, there are big variations between versions of the software. The same version of the browser may not look the same on a PC as it does on a Mac. Also, some people have separate software, such as plug-ins, that provide special capabilities. If you don't know that, it is hard to use any but the most basic web browser capabilities.
Some sites monitor which browsers are being used to view their pages.
Any serious site should probably do the same. You can see from this example, from a University of Illinois Engineering Lab, that more people use Internet Explorer than Netscape. Further, most of the browsers were relatively new. Version 4 of both browsers was a significant upgrade and provides access to a large number of important design features. Whether this is acceptable is an important question for them to answer.