Printed two point type is illegible, except with a strong magnifying glass. It is not possible to form a capital character set less than seven pixels high. If you want lower case too, you need nine pixels top to bottom. The result of having insufficient pixels to describe a character is malformed type, as in the example opposite. This may look quite readable on the designer's screen, on another, it is highly unreadable.
Readability is more to do with the flow, or resistance to flow, of reading text.
Now, having said all this, type readability is relative, not absolute. If everybody designed highly readable black on grey Web pages, life would be very boring indeed. It is not the only issue to be addressed by the Web typographer, just one that needs the application of a little common sense. The Romans and Egyptians invented the letterforms and numerals that we use today and letters that they carved in stone were finished off with a 'serif'. The original purpose of the serif was a practical one, to stop the stone from splitting beyond the main strokes, but serifs also provide a visual barrier to the tops and bottoms of letters.