Designing accessible websites for visually-impaired visitors is not as easy as it should be, but there's still plenty one can do to help. Examples:
- Ensure that all information conveyed with colour is also available without colour, for example from context or markup.
- Make sure your page works when the font size is greatly increased (especially by using a 'liquid' layout that flows to fit the window size and shape, rather than a fixed-size layout).
- Only use tables for tabular information -- don't use tables for layout effects.
The W3C has a downloadable checklist here which lists suggestions in three levels of priority. The designs I supply will meet all the priority 1 requirements and most of the priority 2 requirements.