I coded this script to test my server-side driven RWD websites back in 2011. Thousands of online alternative now exist, this script was created to work offline from my computer.
It also comes with a few nice features like change orientation & scrollbar & and navigation bar.
Just copy/paste this script (and its CSS file) to wherever you want. (yes mate, even there if you want...)
Just type a URL in the topbar, it reloads/updates the frame below as you type.
You can then choose between the few resolutions/devices on the top right. If you want this change to be "permanent" (aka when you hit Cmd+R or F5), click on "OK"
You can swap width/height values by clicking on "rotate"
Any contribution is always welcome.