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HTML

<!DOCTYPE html>
<html>

<head>
<title>Mozilla CSS Getting Started - JavaScript demonstration</title>
</head>

<body>
<h1>JavaScript sample</h1>

<div id="square"></div>

<button type="button" id="clickMe">Click Me</button>

</body>
</html>

CSS

/*** JavaScript demonstration ***/
square {
  width: 20em;
  height: 20em;
  border: 2px inset gray;
  margin-bottom: 1em;
}

button {
  padding: .5em 2em;
}

JavaScript

var square = document.getElementById("square"),
    clickMe = document.getElementById('clickMe'); //Keeping it unobstrusive
function doDemo () {

  var button = this;
  square.style.backgroundColor = "#fa4";
  button.setAttribute("disabled", "true");
  setTimeout(clearDemo, 5000, button);
}

function clearDemo (button) {
  var square = document.getElementById("square");
  square.style.backgroundColor = "transparent";
  button.removeAttribute("disabled");
}

clickMe.onclick = doDemo; //Onclick call. Pass no arguments.