For Lea Verou, with love :-p (response to https://twitter.com/#!/LeaVerou/status/114636985846607874)

gistfile1.js

var keys = [];
var n;
var P = Object.getPrototypeOf;
var o = window;

// Retrieving all property names of the global object,
// because proto chain are messy and different in all browsers
while(o !== null){
  keys = keys.concat(Object.getOwnPropertyNames(o));
  o = P(o);
}

keys = keys.filter(function(e){
                     // A child "class" of HTMLElement has its prototype to HTMLElement.prototype
                     try{ 
                       return P(window[e].prototype) === HTMLElement.prototype;
                     }
                     catch(e){ // window[e] or window[e].prototype is not an object
                       return false;
                     }
                   });

console.log(keys);