Question

As far as I know it’s the ratio between the “abstract” resolution and the device’s physical resolution. So I test it (on HTC Desire), the physical resolution is 480x800, it’s logged out the ratio is 1.5. I threw in some elements, but it still take exactly 480px width to fill the viewport where my naive thought it’s need ‘320px’?

Answer 1

From http://www.quirksmode.org/blog/archives/2012/07/more_about_devi.html, where the author discusses the differences in devicePixelRatio across mobile devices:

On iOS Retina devices, screen.width gives the width in dips. So both a
retina and a non-retina iPad report 768 in portrait mode. On the three
Android devices, screen.width gives the width in physical pixels; 480,
720, and 800, respectively. All browsers on the devices use the same
values. (Imagine if some browsers on the same device used dips and
others physical pixels!)

Which leads the author to the following conclusion:

• On iOS devices, multiply devicePixelRatio by screen.width to get the physical pixel count.
• On Android and Windows Phone devices, divide screen.width by devicePixelRatio to get the dips count.

What matters in your case is screen width, plain and simple. The calculation of DIPs is something for the device to take care of, rather than you as the developer. If the device wants to compensate for a different pixel ratio, it will serve you a width in DIP and give the ratio. If it feels that pages should be displayed with the native unmodified pixel resolution, it will serve you that width instead. The author of the post also comes to the following conclusion which I find interesting:

Apple added pixels because it wanted to make the display crisper and smoother, while the Android vendors added pixels to cram more stuff onto the screen.

At any rate, use the width the browser gives you and leave it to the device to compensate.