简单的样式：

body{
    margin: 0;
    padding: 0;
}
#canvas{
    display: block;
    position: relative;
    margin: auto;
}

创建绘图标签：

<canvas id="canvas">
    您的浏览器不支持，请用升级浏览器或下载chrome浏览器体验
</canvas>

具体实现：

var ball={
    x:200,
    y:200,
    r:150,
    vx:7+Math.random()*5,
    vy:3+Math.random()*5
};
var B = true;
window.onload= function () {
    var canvas=document.getElementById("canvas");
    var context=canvas.getContext(‘2d‘);
    canvas.width=window.innerWidth;
    canvas.height=window.innerHeight;
    if (context){//判断是否支持canvas
        setInterval(function () {
            Draw (context,canvas.width,canvas.height);
            ballMove(canvas.width,canvas.height);
        },50);

        canvas.addEventListener("click", function () {
            var x = event.clientX - canvas.getBoundingClientRect().left;
            var y = event.clientY - canvas.getBoundingClientRect().top;
            var click_this=context.isPointInPath(x,y);
            if ( click_this ){
                B = false;
            }else {
                B = true;
            }
        });
    }else {
        alert("您的浏览器不支持，请用升级浏览器或下载chrome浏览器体验");
    }
};
function ballMove(canvasW,canvasH){
    if ( B ){
        ball.x += ball.vx;
        ball.y += ball.vy;
    }
    if( ball.y >= canvasH-ball.r || ball.y <= ball.r ){
        ball.vy = - ball.vy;
    }
    if( ball.x >= canvasW-ball.r || ball.x <= ball.r ){
        ball.vx = -ball.vx;
    }
    return ball
}

function Draw (context,W,H){
    console.log("还在进行");
    context.clearRect(0,0,W, H);
    context.save();

    context.beginPath();
    context.fillStyle = "black";
    context.fillRect(0,0,W, H);
    context.closePath();

    context.beginPath();
    context.arc(ball.x,ball.y,ball.r,0,Math.PI*2);
    context.fillStyle = "#fff";
    context.fill();
    context.clip();

    var imgs=new Image();
    imgs.src="http://www.mamicode.com/imgs/c1.jpg";
    context.fillStyle = context.createPattern(imgs,"repeat");
    context.fill();

    context.restore();
}

canvas实现“探照灯”共能