People use different computers. PCs may dominate the market, but there's over 5 Million people who use the Internet via a different platform. WebTV is also growing in popularity, especially among the elderly and disabled - two groups who are big consumers on the Internet. Different Computers Break Pages. Compare the picture below - this is the same page as it displays on a PC and Mac.
People use different versions of browsers.
Even different browser versions will display your pages differently. And newer versions aren't always better. People use different screen sizes. A different screen size can dramatically change the way your page appears. The webmaster built the page below to look great on a 1024 x 768 screen size.