Actually, ignore my previous message, I realized the way I was making my test images was wrong and it is infact just the color depth, I made a simple test page here, where you can see the problem if you look at it with both a desktop browser and the N9's browser.

And this problem is completely unrelated to the purple lines you get above dark patches on white background.

Edit:
Just noticed that Opera Mobile (from here) does dither higher depth images before displaying them.