Every web user your web page is different. They have...
- Different size screen.
- Different screen resolution.
- Different window size - their browser may not be taking up the whole screen area.
- Different font size preference.
- Different font typeface preference.
- Different colour preference.
Font preferences may not be for purely aesthetic reasons. The user may have medical reasons to have their fonts at a certain size or colour. The dot pitch of their screen may be such that having the size of fonts and elements linked to a number of pixels may produce a result too large or too small.
To keep this simple here's some dos...
- Make your site take up the whole browser window.
- Make your site only work on a certain size screen.