Be sure to check your pages at all the various screen resolutions. Although only a small percentage of users have their monitors set at 640x480, you will want to make sure your site looks good at that resolution. I design my Web pages at 800x600, which is the average resolution. However, more and more users have their computer monitors set at higher resolutions, such as 1024x728. You will see that your pages will look radically different depending on the resolution.
Browsers are another very important consideration.
Your code needs to be very clean and pretty much flawless to display correctly on Netscape. Internet Explorer is much more forgiving. Netscape, on the other hand, is unassuming. If it doesn't understand your code, it simply will not display it. However, those technologies work better on Internet Explorer than they do on Netscape.