I'd love to see some decent research on this. I'd like to see what the eyesight is like for the average web designer, compared to the average eyesight for the general population.
Here were not talking about people who meet the level of "visual impairment" - they may wear glasses, but their nowhere near meeting requirements to be counted as disabled.
Yet very many websites make the mistake of having either light text on a bright background, or of having a bright white background and black text.
A vision-simulation mode plug in might be useful.