<div> doesn't move

<!doctype html>
<html>
<head>
<meta charset="UTF-8">
<title>TestGame</title>
</head>
<body>
	<div id="Player1" style="background-color:red; height:50px; width:50px;"></div>

<script>
	document.addEventListener('keydown', function(event){
		if (event.keyCode == 65){ //keyCode a
			document.getElementById("Player1").style.left = "10px";
		}
		if (event.keyCode == 68){ //keyCode d
			document.getElementById("Player1").style.left = "-10px";
		}
	});
</script>
</body>
</html>