simple script for showing current screen width so you know where to set a breakpoint in your design

developissimo.js

var developissimo = function() {
	var devcon = document.createElement('div'),
		devstyle = document.createElement('style');

	devcon.id = 'developissimo';
	devstyle.innerHTML = '#developissimo{position:absolute;top:0;right:0;z-index: 10000;padding:5px;background:#fff}';
	document.head.appendChild(devstyle);
	devcon.innerHTML = window.innerWidth;
	document.body.appendChild(devcon);

	window.onresize = resize;

	function resize () {
		console.log('resize');
		devcon.innerHTML = window.innerWidth;
	}
};
developissimo();