使用canvas时需要做兼容性判断，具体方式有2种：
第一种
<canvas>当前浏览器存在兼容性问题</canvas>
第二种
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
</head>
<body>
<canvas id='canvas' width="1024" height="768" style="border: 1px solid #aaa;display: block;margin: 5px;"></canvas>
<script>
window.onload=function() {
var canvas=document.getElementById('canvas');
canvas.width=1024;
canvas.height=768;
var context=canvas.getContext("2d");
if(canvas){
//....
}else{
alert("当前浏览器存在兼容性问题")
}
}
</script>
</body>
</html>
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