最先绘制的黑色背景怎么不显示了，求老师解答下 ，谢谢了
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<html>
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
*{margin:0;}
#canvas{margin:0 auto;display: block; background: #ccc;}
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<body>
<canvas id="canvas" width="600" height="600"></canvas>
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var canvas=document.getElementById('canvas');
var cxt=canvas.getContext('2d');
cxt.save();
cxt.beginPath();
cxt.fillStyle="#000"
cxt.fillRect(0,0,600,600);
cxt.beginPath();
cxt.fillStyle="#f00"
cxt.fillRect(0,0,600,600);
cxt.globalCompositeOperation="destination-out";
cxt.beginPath();
cxt.fillStyle="#00f"
cxt.fillRect(200,200,200,200);
cxt.beginPath();
cxt.arc(200,200,50,0,2*Math.PI);
cxt.fillStyle="#0f0";
cxt.fill();
cxt.beginPath();
cxt.fillStyle="#00f"
cxt.fillRect(400,400,100,100);
cxt.restore();
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