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<!DOCTYPE html>
<html>
<body>

<div id="myDiv" style="border-bottom:thick solid red">This is a div element.</div>
<br>
<button type="button" onclick="myFunction()">Return the width of the bottom border</button>

<script>
function myFunction() {
    alert(document.getElementById("myDiv").style.borderBottomWidth);
}
</script>

</body>
</html>