There are two definitions of a pixel, one is the hardware pixel or the size of one unit on a physical screen, the other and w3c standard is a relative size based on a reference pixel.
How big is a pixel exactly according to the spec?
??For a nominal arm’s length of 28 inches, the visual angle is therefore about 0.0213 degrees??. ??For reading at arm’s length??, ??1px thus corresponds to about 0??.??26 mm?? (??1/96 inch??).
That probably doesn’t give you a good idea as to how the unit behaves. So imagine looking up at the moon, then bringing your thumb up and covering the moon. Optically your thumb and the moon are the same size, but physically the moon is much larger than your thumb.
When dealing with font hinting and anti-aliasing you are altering how text falls across various hardware pixels, not reference pixels. A bitmap font scaled on some screens to the w3c standard of a pixel might have issues displaying properly. Using the method outlined in the article can work to fix this or using the method “described earlier in the comments”:http://www.alistapart.com/comments/a-pixel-identity-crisis/P10/#13 of using target-densitydpi=device-dpi might help with bitmap fonts.