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Edit This Code:
<!DOCTYPE html>
<html>
<head>
<style>
#myDiv {
    border: 1px solid #FF0000;
}
</style>

<script>
var _hmt = _hmt || [];
(function() {
var hm = document.createElement("script");
hm.src = "//hm.baidu.com/hm.js?73c27e26f610eb3c9f3feb0c75b03925";
var s = document.getElementsByTagName("script")[0];
s.parentNode.insertBefore(hm, s);
})();
</script>
</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>