If the pixel density of the output device is very different from that of a typical computer display, the user agent should rescale pixel values. It is recommended that the pixel unit refer to the whole number of device pixels that best approximates the reference pixel. It is recommended that the reference pixel be the visual angle of one pixel on a device with a pixel density of 96dpi and a distance from the reader of an arm's length.Using the viewport meta tag to control layout on mobile browsers
If you're designing flexibly, use the viewport meta tag in your <head>.Quick Tip: Don’t Forget the Viewport Meta Tag
