<!DOCTYPE html>
<html>
<head>
<style>
div::first-letter {
font-size: xx-large;
}
</style>
</head>
<body>
<p>Click the button to get the computed font size for the first letter in the test div.</p>
<p><button onclick="myFunction()">Try it</button></p>
<div id="test" style="height: 50px;background-color: lightblue;">Test Div</div>
<p>The computed font size for div::first-letter in the test div is: <span id="demo"></span></p>
<script>
function myFunction(){
var elem = document.getElementById("test");
var theCSSprop = window.getComputedStyle(elem, "first-letter").getPropertyValue("font-size");
document.getElementById("demo").innerHTML = theCSSprop;
}
</script>
</body>
</html>