font size chage

<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>글자크기</title>
	<style>
		p {
			font-size: 1.0em;
		}
		strong {
			font-size: 1.5em;
		}
	</style>
	<script>
		var dfs = 1.0;
		function changeSize(s) {
			var obj = document.getElementById("mytext");
			dfs += s;
			obj.style.fontSize = dfs + "em";
		}
	</script>
	<link rel="stylesheet" href="http://code.jquery.com/ui/1.12.1/themes/start/jquery-ui.css">
	<script src="http://code.jquery.com/jquery-1.12.4.min.js"></script>
	<script src="http://code.jquery.com/ui/1.12.1/jquery-ui.min.js"></script>
</head>
<body>
	<button onclick="changeSize(0.1)">+</button>
	<button onclick="changeSize(-0.1)">-</button>
	<form>
		<input type="text" id="date" name="date">
	</form>
	<p id="mytext"><strong>Lorem ipsum</strong> dolor sit amet, consectetur adipisicing elit. Officiis, aperiam, veritatis! Officiis harum id ratione aperiam veritatis quam, et necessitatibus consequuntur enim eum repellat, minus dolore, corrupti, omnis doloremque consequatur.</p>
	<script>
		$("#date").datepicker({
			dateFormat: "yy-mm-dd",
			minDate:-3,
			maxDate:3
		});
	</script>
</body>
</html>