CSS：
<div class="circle"> <p><span id="loadedNum">0</span>%</p> <div class="bg" id="bg"></div> <canvas class="mask" width="204" height="204" ></canvas> </div>
HTML:
<div class="circle">
<p><span id="loadedNum">0</span>%</p>
<div class="bg" id="bg"></div>
<canvas class="mask" width="204" height="204" ></canvas>
</div>
JS：
function inte(percent) {
if (percent < 0 || percent > 100) {
throw new Error('percent must be between 0 and 100');
return
}
var canvas = document.querySelector('canvas');
var ctx = canvas.getContext('2d');
ctx.lineWidth = 10;
ctx.strokeStyle = "red";
var angle = 0;
var timer;
(function draw() {
timer = requestAnimationFrame(draw);
ctx.beginPath();
ctx.arc(canvas.width / 2, canvas.height / 2, 100, 0, angle * Math.PI / 180, false);
angle++
var percentAge = parseInt((angle / 360) * 100)
if (angle > (percent / 100 * 360)) {
percentAge = percent
window.cancelAnimationFrame(timer);
};
document.querySelector('#loadedNum').innerHTML = percentAge;
ctx.stroke();
})()
}
window.onload = inte(20)
DEMO地址：http://codepen.io/jonechen/pen/vGWZQW
上面这个demo已经实现了百分比圆环进度，但是不足的是，使用了div和canvas两个元素的组合，可以再进化一下，HTML部分去掉div，都使用canvas来实现；见代码：
CSS：
*{padding: 0; margin: 0; }
.circle{ 200px;height: 200px;margin: 20em auto;position: relative;}
canvas{display: block;margin: 0;position: absolute;background: white;left: 0;top: 0;}
#canvas_1{z-index: 1 }
#canvas_2{z-index: 2; background: transparent;transform:rotate(-90deg); }
HTML：
<div class="circle">
<canvas id="canvas_1" width="200" height="200"></canvas>
<canvas id="canvas_2" width="200" height="200"></canvas>
</div>
JS：
function inte(percent) {
var canvas_1 = document.querySelector('#canvas_1');
var canvas_2 = document.querySelector('#canvas_2');
var ctx_1 = canvas_1.getContext('2d');
var ctx_2 = canvas_2.getContext('2d');
ctx_1.lineWidth = 10;
ctx_1.strokeStyle = "#ccc";
//画底部的灰色圆环
ctx_1.beginPath();
ctx_1.arc(canvas_1.width / 2, canvas_1.height / 2, canvas_1.width / 2 - ctx_1.lineWidth / 2, 0, Math.PI * 2, false);
ctx_1.closePath();
ctx_1.stroke();
if (percent < 0 || percent > 100) {
throw new Error('percent must be between 0 and 100');
return
}
ctx_2.lineWidth = 10;
ctx_2.strokeStyle = "#f90";
var angle = 0;
var timer;
(function draw() {
timer = requestAnimationFrame(draw);
ctx_2.clearRect(0, 0, canvas_2.width, canvas_2.height)
//百分比圆环
ctx_2.beginPath();
ctx_2.arc(canvas_2.width / 2, canvas_2.height / 2, canvas_2.width / 2 - ctx_2.lineWidth / 2, 0, angle * Math.PI / 180, false);
angle++;
var percentAge = parseInt((angle / 360) * 100)
if (angle > (percent / 100 * 360)) {
percentAge = percent
window.cancelAnimationFrame(timer);
};
ctx_2.stroke();
ctx_2.closePath();
ctx_2.save();
ctx_2.beginPath();
ctx_2.rotate(90 * Math.PI / 180)
ctx_2.font = '30px Arial';
ctx_2.fillStyle = 'red';
var text = percentAge + '%';
ctx_2.fillText(text, 80, -90);
ctx_2.closePath();
ctx_2.restore();
})()
}
window.onload = inte(60);