Contrast is the main factor in whether or not text is easy to read. Good contrasts will make text easy on the eyes, easy to scan quickly, and overall more readable. On the other hand, poor contrast will force the user to squint and make reading the body text almost painful, not to mention a lot slower.
Black on white is very readable. Black on white is clearly the standard in contrast colors. For readable content it is good to stay in the range of black-on-white contrast.
However, something like purple on blue, is nearly impossible to read. Poor contrast can have a major impact on the text. You probably won’t see websites using this type of contrast, but you need to be aware of it.
Line height is a very common term meaning the space between individual lines of text. Line height is another factor in the readability of body text and even headers. Sufficient line height is especially important in Web design because it makes the text easier to scan … and scan we do! Line height that is too short will cause users to squint while reading. If it is too large, the text will seem like separate bodies instead of grouped together as one.
Like line height, letter spacing affects readability in Web typography. Letter spacing is the space between each letter in words. Letter spacing is an obvious factor in legibility.
Line length is often overlooked in Web typography but should not be. Line length is, of course, the number of words per line. A good line length is one that allows the reader’s eyes to flow from the end of one line to the beginning of the next very easily and naturally. Aim for a maximum length of around 80 to 100 characters per line, if possible.
For more on web typography readability visit Smashing Magazine’s article, 10 Principles Of Readability And Web Typography