You don't know what browser they're using, and thus what HTML features they can see. Screen display is poorer than print publications, so high resolution graphics are wasted. It is harder to read text on screen and users are more prone to scan and skip about. You need to consider all of these factors, and more, in web design. Good web sites load quickly.
Users are impatient beasts.
Most users are willing to wait 10 to 15 seconds for a page to download, but not much more unless it's a page they really want. Slow download time is the number one complaint of users. Good web sites present their meat in the first one or two screens. Users don't like to scroll. Make the top part of your page count.