keys.html

<html>
<head>
 <title>Key presses</title>
 <style>
  input {
   width: 500px;
  }
 </style>
 <script>
  'use strict';
  function translate(key) {
   if (key === ' ') {
    return 'Space';
   }
   if (key.length === 1) {
    return key.toUpperCase();
   }
   return key;
  }
  function keydown(event) {
   event.preventDefault()
   var key = translate(event.key);
   var text = event.target.value;
   var words = text.split(" ");
   var i;
   for (i in words) {
    var w = words[i];
    if (w === key) {
     return false;
    }
   }
   if (text === "") {
    text = key;
   } else {
    text = text + " " + key
   }
   event.target.value = text
   return false;
  }
  function keyup(event) {
   var key = translate(event.key);
   var text = event.target.value;
   var words = text.split(" ");
   var result = [];
   var i;
   for (i in words) {
    var w = words[i];
    if (w !== "" && w !== key) {
	 result.push(w);
	}
   }
   event.target.value = result.join(" ");
   return false;
  }
 </script>
</head>
<body onload="document.getElementById('input').focus()">
 <h1>Press some keys!</h1>
 <p>Ensure JavaScript is enabled and that the focus is in the input box below, then you can test your keys and key combinations.</p>
 <input id="input" onkeydown="keydown(event)" onkeyup="keyup(event)" />
</body>
</html>