<![CDATA[
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
  "http://www.w3.org/TR/html4/strict.dtd">
<html>
<body>

<script>
function add() {
  google.calendar.addCalendar("en.usa#holiday@group.v.calendar.google.com",
      "US Holidays");
}
</script>

<center>
<button onclick="add()">Add US Holidays</button>
</center>

</body>
</html>
]]>