为什么这不起作用？
<html>
<head>
<title>Learning the basics of canvas</title>
<meta charset="utf-8">
<script type="text/javascript" src="http://ajax.googleapis.com?/ajax/libs/jquery/1/jquery.min.js"></script>
<script type="text/javascript">
$(document).ready(function() {
var canvas = $("#myCanvas");
var context = canvas.get(0).getContext("2d");
context.fillRect(40, 40, 100, 100);
});
</script>
</head>
<body>
<canvas id="myCanvas" width="500" height="500">
<!-- Insert fallback content here -->
</canvas>
</body>
</html>