Serious question. Which is easier on the eyes (less straining for long periods of time).

Black text on white background. OR
White text on black background?

@Ian_Fry I'm pretty sure I've seen scientists claim white text on black is less strenuous, but don't take my word for it.
Even better: Black or dark text on light grey/beige. Too much contrast can be tough too, I've heard.
@zatnosk I'm not sure what studies say, but I find light text on dark backgrounds hard to read. Grayish colors are even worse. I've been known to highlight the text on a page, then paste into a text editor so I can read light-gray on dark-gray or the reverse.
@lnxw48a1 in Firefox you can also try:
view → page style → no style
to see the text without weird colours (also without formatting)