Written in 1948, Rudolf Flesch’s book The Art of Readable Writing takes on some typography rules near the end. These could have been written this morning, and they apply to writing for the Web as much as they applied to writing 70 years ago.
1) Any type size under 10 point is hard to read.
2) Anything printed in an unfamiliar typeface is hard to read.
3) If there is no leading (by leading he means white space between lines), lines longer than 40 characters are hard to read. The rule of thumb is that “one-and-half alphabets” or 39 characters and wider require increased spacing between lines.
4) Headings printed in CAPITALS only are hard to read, especially if they are BOLD.
5) Try to arrange the page so to make reading and understanding easier. Use bullet items, indentation and shorter paragraphs.