案例：html/html5 案例     状态：可编辑再运行    进入竖版
 运行结果　
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<!DOCTYPE html>
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<html>
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<body>
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​
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<canvas id="myCanvas" width="300" height="150" style="border:1px solid #d3d3d3;">
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Your browser does not support the HTML5 canvas tag.
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</canvas>
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​
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<script>
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​
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var c=document.getElementById("myCanvas");
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var ctx=c.getContext("2d");
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ctx.fillStyle="red";
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ctx.fillRect(20,20,75,50);
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​
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//Turn transparency on
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ctx.globalAlpha=0.2;
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ctx.fillStyle="green";
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ctx.fillRect(50,50,75,50);
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ctx.fillStyle="blue";
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ctx.fillRect(80,80,75,50);
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</script>
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​
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</body>
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</html>
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​