<!DOCTYPE HTML>
<html>
<head>
<meta charset="utf-8">
<style>
#pen {
height:100px;
background-color:red;
}
</style>
</head>
<body>
<div id = "pen">
</div>
<script> // это <script src="testJScript.js">
var pen = document.getElementById("pen")
pen.style.height = "300px";
alert(pen.style.height);
</script>
</body>
</html>