From http://www.w3schools.com (Copyright Refsnes Data)

Web Quality - Readability

Proper use of fonts and colors will make your website easier to read.

Mind the Color Contrasts

Black text on a white background, or white text on a black background, is best for people with viewing disabilities, and for display on bad equipment.

Grey text on a light background can give a very poor contrast:

Grey text on a dark background can also give a very poor contrast:

Some combinations - like black and red, black and blue, yellow and green - always strain the eye:

And some combinations are not so bad:

Mind Your Letter Spacing

Text with close letter spacing presents special difficulties for readers who are visually impaired.

Text with extra letter spacing is easy to read

Text with reduced letter spacing is difficult to read

Mind Your Line Spacing

Text with good line spacing
is easy to read

Text with reduced line spacing
is difficult to read

Avoid the Fancy Fonts

This font is easy to read

This font is not

Speak Less Italian

Normal fonts are easy to read. Normal font is easy to read. Normal fonts are easy to read. Normal font is easy to read. Normal fonts are easy to read.

Italic fonts are not so easy to read. Italic font is not so easy to read. Italic fonts are not so easy to read. Italic font is not so easy to read.

From http://www.w3schools.com (Copyright Refsnes Data)