Based on https://gist.github.com/vasilisvg/e79dae967e25d3ce1d64

list-chars.js

void function() {
	var chars = document.body.innerText.split('').sort().filter(function(char, i, arr) {
		return i === 0 || arr[i - 1] !== char;
	}).join('');
	var textarea = document.createElement('textarea'),
	    snippet = '<pre><link rel="stylesheet" type="text/css" href="http://fonts.googleapis.com/css?family=Inconsolata&text=' + chars + '"></pre>';
	
	textarea.innerHTML = snippet;
	textarea.setAttribute('style','position:fixed;top:5vh;left:5vw;height:90vh;width:90vw;box-shadow:0 0 5vmax black;background:white');
	document.body.appendChild(textarea);
}();