求助！ 画布无法在适应在浏览器正中间
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>tinyHeart</title>
<style type="text/css">
body {
padding-top:10px;
}
.all_bg{
width:800px;
height:600px;
margin: 0px auto;
}
#allconvas{
position: relative;
width: 800px;
height: 600px;
margin: 0px;
}
#canvas1{
position: absolute;
bottom: 0;
left: 0;
z-index: 1;
}
#canvas2{
position: absolute;
bottom: 0;
left: 0;
z-index: 0;
}
</style>
</head>
<body>
<div calss="all_bg">
<div id="allcanvas">
<canvas id="canvas1" width="800" height="600"></canvas>
<canvas id="canvas2" width="800" height="600"></canvas>
</div>
</div>
</body>
</html>一直显示在浏览器左下角，而且ui设置应该是画布正中才对，显示却偏右..
