Question

@media only screen and (max-device-width:480px), 
only screen and (-webkit-min-device-pixel-ratio: 2)
{ /*iphone css*/ }

max-device width makes sense but pixel ratio doesn’t make any sense to me. thanks.

Answer 1

From http://developer.android.com/reference/android/webkit/WebView.html

The -webkit-device-pixel-ratio CSS media query. Use this to specify
the screen densities for which this style sheet is to be used. The
corresponding value should be either “0.75”, “1”, or “1.5”, to
indicate that the styles are for devices with low density, medium
density, or high density screens, respectively. For example: The hdpi.css stylesheet is only used for devices
with a screen pixel ration of 1.5, which is the high density pixel
ratio.

And according to https://developer.mozilla.org/en/CSS/Media_queries

-moz-device-pixel-ratio

Gives the number of device pixels per CSS pixel.

Note: This media feature is also implemented by Webkit as
-webkit-device-pixel-ratio. The min and max prefixes as implemented by Gecko are named min–moz-device-pixel-ratio and
max–moz-device-pixel-ratio; but the same prefixes as implemented by
Webkit are named -webkit-min-device-pixel-ratio and
-webkit-max-device-pixel-ratio.