<!DOCTYPE html>
<html>
<head>
<style>
#myDiv {
border: 1px solid #FF0000;
}
</style>
</head>
<body>
<div id="myDiv" style="margin-bottom:5cm;">This is a div.</div>
<br>
<button type="button" onclick="myFunction()">Return bottom margin</button>
<script>
function myFunction() {
alert(document.getElementById("myDiv").style.marginBottom);
}
</script>
</body>
</html>