Screentest.html will help you to test resolutions on different devices. You will be able to better understand behavior of breakpoints on different devices/screens.
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It shows info about screen: CSS resolution, device screen ratio, physical resolution.
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It displays range in which you are (in terms of breakpoint's ranges).
Sometimes it useful to see what resolution do you have on your devices. What breakpoints will you need for this device. How do you need to work with it.
This simple html with small JS helps. Just put this page somewhere on the web and open on your device.
You could set an array of breakpoints. There are three points and four ranges.
Examples:
var breakpoints = [768, 1024, 1256];
In such case there are 4 ranges: 0 - 768, 768 - 1024, 1024 - 1256, 1256+
For Bootstrap v4:
// Bootstrap v4 alpha, http://v4-alpha.getbootstrap.com/layout/overview/
// Without Extra small devices (543px)
var breakpoints = [767, 991, 1199];
For Desktops you can resize your screen and see what range do you have right now.