Thankful Eyes is a bookmarklet which automatically calculates and applies a large enough font size to body text for optimum readability.
Click your bookmarklet button on any page that has illegibly small body text. To undo the changes made by Thankful Eyes, refresh the page.
You can adjust two variables at the top of the script to control its behaviour.
Thankful Eyes is developed on and for Opera. However, it works as-is on Firefox, too.
It is said that, for optimum readability, text line length should be around 12 words per line. This allows the eye to easily track from the end of one line to the beginning of the next.
Using Thankful Eyes is better than simply maintaining a minimum font size with your browser settings because doing that increases the size of many elements which usually do not require magnification, such as sidebar text, menu items, small labels, and so on. Thankful Eyes aims to adjust only body text.
Thankful Eyes is better than using browser zoom, because browser zoom also magnifies and distorts images.