"My eyes are happiest when"...it really depends...<p>To use an extreme example, back when I was experiencing corneal erosions (you don't want those), contrast itself was the enemy. You send me this website, I am going to have to play with my display settings and also probably put on sunglasses.<p>Thinking of more common contextual preference change, time of day comes to mind.<p>I wish my e-reader clients could pick up on my time-of-day settings changes, because sometimes I want contrast and white on black, just dimmed in general, and at other times I want sepia but brighter...but generally there are obvious rules that could be derived based on what time it is.<p>(Sending people to websites like this without caging it in some kind of wink or chuckle of an expression seems like it would be a bit over the top...)