<html>
<head>
<script type="text/javascript">
var canvas=document.getElementById("screen");
var ctx=canvas.getContext("2d");
ctx.fillStyle="#f00";
ctx.fillRect(30,40,80,100);
</script>
</head>
<body οnlοad="onReady()">
<canvas id="screen" width="600" height="400">
</canvas>
</body>
</html>
<html>
<head>
<script type="text/javascript">
function onReady(){
var canvas=document.getElementById("screen");
var ctx=canvas.getContext("2d");
ctx.fillStyle="#f00";
ctx.fillRect(30,40,80,100);
}
</script>
</head>
<body οnlοad="onReady()">
<canvas id="screen" width="600" height="400">
</canvas>
</body>
</html>看來就是這個問題，要在頁面載入後才能獲得canvas對象，這個問題在firefox不會出現，看來是做了優化，反正還是注意吧，小問題