<head>
<meta charset="UTF-8">
<title>canvas_learn</title>
<style>
#canvas {
box-shadow: 2px 2px 2px 2px #888888;
position: absolute;//绝对定位
top: 0;
bottom: 0;
left: 0;
right: 0;
margin: auto;//垂直居中
}
</style>
</head>
<body>
<canvas id="canvas" height="600" width="700">helloha!!!</canvas>
</body>
</html>