There is much to do over responsive design, a technique to shape websites for for example a mobile phone. Or a television. Or a fridge.
At my job we use mostly the max-width media query.
With mobile phones there are several operating systems and browsers, and a pixel in hardware is not the same as a css-pixel. To simply show this I wrote a script so you can view which device has which viewport.
You can see the output of the script. You can also view the source code.