console
var canvas = document.getElementById('canvas'),
context = canvas.getContext('2d'),
centerX = canvas.width/2,
centerY = canvas.height/2,
rad = Math.PI*2/100,
speed = 0.1;
function blueCircle(n){
context.save();
context.beginPath();
context.strokeStyle = "#49f";
context.lineWidth = 12;
context.arc(centerX, centerY, 100 , -Math.PI/2, -Math.PI/2 + n*rad, false);
context.stroke();
context.restore();
}
function whiteCircle(){
context.save();
context.beginPath();
context.strokeStyle = "#A5DEF1";
context.lineWidth = 12;
context.arc(centerX, centerY, 100 , 0, Math.PI*2, false);
context.stroke();
context.closePath();
context.restore();
}
function text(n){
context.save();
context.fillStyle = "#F47C7C";
context.font = "40px Arial";
context.textAlign = "center";
context.textBaseline = "middle";
context.fillText(n.toFixed(0)+"%", centerX, centerY);
context.restore();
}
(function drawFrame(){
window.requestAnimationFrame(drawFrame, canvas);
context.clearRect(0, 0, canvas.width, canvas.height);
whiteCircle();
text(speed);
blueCircle(speed);
if(speed > 100) speed = 0;
speed += 0.1;
}());
<div >
<h1 align='center' style="color:red;">页面效果练习</h1>
<h2 align='center' style="color:blue;">1.canvas动画</h2>
</div>
<canvas id="canvas" width='500' height="500">
</canvas>