Accessibility is the key to maximising the number of users who can effectively use your website.
Use of appropriate foreground / background colour combinations
Reducing dependence on specific screen size, allowing the user to decide the size of the screen and its position
Removing dependence on specific text size, allowing the website to operate with the text size the user wants
Balanced use of graphics with meaningful alternate names
Placement of menus and text such that "screen readers" operate correctly
Where applicable allowing the user to set defaults to ease their use (eg colour selections, metric versus imperial units etc)