Tryit Editor v1.8

Source Code:

<!DOCTYPE html>
<html>
<head>
<script>
function ChangeOpacity(x)
{
// Return the text of the selected option
var opacity=x.options[x.selectedIndex].text;
var el=document.getElementById("p1");
if (el.style.opacity!==undefined)
  {el.style.opacity=opacity;}
else
  {alert("Your browser doesn't support this example!");}
}
</script>
</head>
<body>

<p id="p1">Select a value from the list below, to change this element's opacity!</p>
<select onchange="ChangeOpacity(this);" size="5">
  <option />0
  <option />0.2
  <option />0.5
  <option />0.8
  <option selected="selected" />1
</select>

</body>
</html>

Result:

W3Schools.com - Try it yourself

Edit the code above and click "Submit Code" to see the result.