效果出不来
var dom = document.getElementById("clock");
var ctx = dom.getContext('2d');
var width = ctx.canvas.width;
var height = ctx.canvas.height;
var r = width/2;
function drawBackground(){
ctx.translate(r,r);
ctx.beginPath(); //起始路径
ctx.lineWidth = 10;
ctx.arc(0, 0, r-5, 0, 2*Math.PI, false);
ctx.stroke(); //绘制圆 fill（）填充圆
}
drawBackground();
