1. Do not use absolute font sizes in your style sheets. Code font sizes in relative terms, typically using percentages such as 120% for big text and 90% for small text.
2. Make your default font size reasonably big (at least 10 point) so that very few users have to resort to manual overrides.
3. If your site targets senior citizens, use bigger default font sizes (at least 12 point).
4. If possible, avoid text that’s embedded within a graphic, since style sheets and font size buttons don’t have any effect on graphics. If you must use pictures of text, make sure the font size is especially large (at least 12 point) and that you use high-contrast colors.
5. Consider adding a button that loads an alternate style sheet with really big font sizes if most of your site’s visitors are senior citizens or low-vision users. Few users know how to find or use the built-in font size feature in current browsers, and adding such a button within your pages will help users easily increase text size. However, because every extra feature takes away from the rest of the page, I don’t recommend such a button for mainstream websites.
6. Maximize the color contrast between the text and the background (and do not use busy or watermarked background patterns). Despite the fact that low-contrast text further reduces readability, the Web is plagued by gray text these days.