Grab and list all (unique) classes on page

gistfile1.js

var cls = [];
Array.prototype.slice.call( document.querySelectorAll('*'), 0 ).forEach( function ( el ) {
	cls = cls.concat( Array.prototype.slice.call( el.classList, 0 ) );
} );
cls = cls.reduce( function ( p, c ) {
	if ( p.indexOf( c ) < 0 ) p.push( c );
	return p;
}, [] );
console.log( cls );