正方形边框的结果显示出不同的宽度，似乎右侧和底部边框的宽度是左侧和顶部边框宽度的两倍。为什么会这么奇怪？我想让所有边框的宽度都相同。
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>HTML5 Test</title>
<script type="text/javascript">
function draw () {
var canvas = document.getElementById('rectangle');
var ctx = canvas.getContext('2d');
ctx.save();
ctx.lineWidth = 30;
ctx.fillStyle = "black";
ctx.fillRect(0,0,100,100);
ctx.strokeStyle = "red";
ctx.strokeRect(0,0,100,100);
ctx.restore();
}
</script>
</head>
<body onload="draw()">
<canvas id="rectangle"></canvas>
</body>
</html>
