Monitors:
Monitors make up the majority of variables in the appearance of a Web site. If all monitors were the same size, resolution and registration it would be much easier to predict how a Web site would look. Alas, they are not.

On top of the actual size of a screen, the resolution of the sceen must also be considered. Resolution is measured in pixels per inch (ppi). Larger monitors have viewing areas of up to 1920 x 1080 ppi. They also usually have the ability to be switched to several settings lower. A web site will appear larger at a lower setting than at the higher settings. The most commonly used resolution is the 800 x 600 setting and is usually the best size to design a web site for, depending on the needs of the site.

Quality of images in the site depends on monitor type too. Your basic office work monitor has much less capability to display a graphic accurately than say a graphic designer's monitor, and is less likely to be adjusted correctly as well. Also, since IBM monitors uses what is called "SRGB" (small RGB) which reduces the amount of information each pixel gets, graphics appearing on them will not look as good as on a Mac monitor.