分辨访问器是不是适用canvas，实际编码以下：
<!DOCTYPE html>
<head>
<meta charset=utf⑻ />
<title>canvas</title>
<script type='text/javascript'>
window.onload = function(){
/**分辨访问器是不是适用canvas**/
try{
document.createElement('canvas').getContext('2d');
var addDiv = document.createElement('div');
addDiv.innerHTML='适用canvas';
document.body.appendChild(addDiv);
}catch(e){
var addDiv = document.createElement('div');
addDiv.innerHTML='访问器不适用canvas';
document.body.appendChild(addDiv);
}
};
</script>
</head>
<body>
</body>
</html>
