CSS pixel & Device pixel

1.
At zoom level 100% one CSS pixel is exactly equal to one device pixel.

2.
Zooming as implemented in modern browsers consists of nothing more than “stretching up” pixels. That is, the width of the element is not changed from 128 to 256 pixels; instead the actual pixels are doubled in size. Formally, the element still has a width of 128 CSS pixels, even though it happens to take the space of 256 device pixels.

以上節錄自http://www.quirksmode.org/