Use device pixel ratios when defined instead of calculating, fix #521

[maikelvl]

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[ai]

Men, I told you, that min-device-pixel-ratio in wrong way ;). There is not min-device-pixel-ratio in spec. There is only min-resolution.

So this code has no sence:

@media (min-device-pixel-ratio: 2), (min-resolution: 192dpi) { }

Only this code is right:

@media (min-resolution: 2dppx) { }

Look at dppx unit. It is exactly same thing as device-pixel-ratio. dp (device pixel) per px (pixel).

[ai]

I asked you about round dpi to min-device-pixel-ratio calculations.

[maikelvl]

Aha, I see. Now I know why this issue occurred: 350dpi != 3dppx
That code was used in a tutorial (http://www.sitepoint.com/watch-creating-high-resolution-background-images-with-css/)

Nevermind merging, I get it ;)
Thanks