JS: Show viewport size for easy responsive development (no need to have the dev tools open).

viewport-size.js

// Show viewport size
document.addEventListener('DOMContentLoaded', function() {
	document.body.insertAdjacentHTML('afterend', '<div id="viewport-size" style="position:fixed; top:0; right:0; z-index: 9999; padding:0 .5em; color:#333; background:rgba(255,255,255,.75); font: 14px/1.8em Monaco, Inconsolata, Ubuntu Mono, Courier New, monospaced;"></div>');
	showViewportSize();
});

window.onresize = function() { showViewportSize(); }

function showViewportSize() {
	var body   = document.body,
	    html   = document.documentElement,
	    width  = Math.max( html.clientWidth, window.innerWidth ),
	    height = Math.max( body.scrollHeight, body.offsetHeight, html.clientHeight, html.scrollHeight, html.offsetHeight );
	document.getElementById('viewport-size').innerHTML = width + ' × ' + height;
}

O Paulo Diovani fez uma versão melhor e muito mais elegante.