<!DOCTYPE html>
<html>
<head>
<style>
#myDiv {
border: 1px solid red;
outline: dotted green;
}
</style>
</head>
<body>
<div id="myDiv" style="outline-width:10px;">This is a div element.</div>
<br>
<button type="button" onclick="myFunction()">Return outline width</button>
<script>
function myFunction() {
alert(document.getElementById("myDiv").style.outlineWidth);
}
</script>
</body>
</html>