<canvas id="myCanvas" width="500" height="500" style="border:1px solid #d3d3d3;">

您的浏览器不支持 HTML5 canvas 标签。</canvas>

<script>

var c=document.getElementById("myCanvas");

var ctx=c.getContext("2d");

ctx.beginPath();

ctx.arc(250,250,250,0,Math.PI);

ctx.stroke();

</script>