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. Unfortunately the important map on the right of the page is off the screen when viewed on a smaller screen size. Before any webmaster publishes a Web page, that page should be checked on all the major browsers and computers. Failure to create a compatible Web page can cause visitors to leave your site - never to return.