要使用HTML5 Canvas创建椭圆，我们可以保存上下文状态，水平拉伸画布上下文，绘制圆形，恢复画布状态，然后应用样式。
html代码：
<!DOCTYPE HTML>
<html>
<head>
<style>
body {
margin: 0px;
padding: 0px;
}
</style>
</head>
<body>
<canvas id="myCanvas" width="578" height="200"></canvas>
<script>
var canvas = document.getElementById('myCanvas');
var context = canvas.getContext('2d');
var centerX = 0;
var centerY = 0;
var radius = 50;
// save state
context.save();
// translate context
context.translate(canvas.width / 2, canvas.height / 2);
// scale context horizontally
context.scale(2, 1);
// draw circle which will be stretched into an oval
context.beginPath();
context.arc(centerX, centerY, radius, 0, 2 * Math.PI, false);
// restore to original state
context.restore();
// apply styling
context.fillStyle = '#8ED6FF';
context.fill();
context.lineWidth = 5;
context.strokeStyle = 'black';
context.stroke();
</script>
</body>
</html>
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