Actually, on a screen black and white is not at all easy to read.postby debunix » Mar 16th, '16, 23:57
Black on white is best for reading, black on grey is harder for those with less perfect eyes
The contrast of glowing white (from the screen)) and deep black is too strong and creates a flickering which is not very good for ones eyes.
That is why most serious webdesigners will never use a pure white background but a very very light grey, or a dark grey for the text. That is easier on the eyes.
Apart from that legibility depneds on many more factors than just colour.
Typeface, typesizes, weight of the type, spacing of letters and interlignage are all equally important.
Not as easy as black and white