window.onload = function(){

  var a = [];

  var log = document.getElementById("keys");

  document.body.onkeydown = function(event){

    var keyCode = ('which' in event) ? event.which : event.keyCode;

    if(a.indexOf(keyCode) == -1) a.push(keyCode);

    log.innerHTML = a.length + " | " + a.join();

    return false;

  };  

  document.body.onkeyup = function(event){

    a = [];

    log.innerHTML = a.join();

  };

​

};