代码如下:

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<!doctype html>
<html>
    
<head>
    <meta charset="utf-8">
    <title>星空背景代码实现</title>
    <script src="https://libs.baidu.com/jquery/1.11.3/jquery.min.js"></script>
    <style type="text/css">
        #canvas{
            position: absolute;
            z-index: -1;
            left: 0;
            top: 0;
        }
        .main{
            position: absolute;
            z-index: 2;
            top: 0;
            left: 0;
            
        }
        h1{
            color: #FFFFFF;
        }
    </style>
</head>

<body>
    <div id="main">
        <h1>canvas</h1>
    </div>
    <!-- 
        1、简要介绍 canvas 绘制  js代码     
        2、如何用来做背景   z-index 什么情况下才有用
        3、弹性盒子作浮动元素居中 display: flex; align-items: center; justify-content: center;
        4、下载网址  https://www.jq22.com/
     -->
    <canvas id="canvas"></canvas>

    <script>
        var canvas = document.getElementById('canvas'),
            ctx = canvas.getContext('2d'),
            w = canvas.width = window.innerWidth,
            h = canvas.height = window.innerHeight,

            hue = 217,      // 十六进制颜色
            stars = [],
            count = 0,
            maxStars = 1200;    // 星星的数量

        var canvas2 = document.createElement('canvas'),
            ctx2 = canvas2.getContext('2d');
        canvas2.width = 100;
        canvas2.height = 100;
        var half = canvas2.width / 2,
            // 绘制一个矩形,并用放射状/圆形渐变进行填充:(x0,y0,r0,x1,y1,r1)
            gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half);
        // gradient.addColorStop(stop 0~1,color);
        gradient2.addColorStop(0.025, '#fff');
        gradient2.addColorStop(0.1, 'hsl(' + hue + ', 61%, 33%)');
        gradient2.addColorStop(0.25, 'hsl(' + hue + ', 64%, 6%)');
        gradient2.addColorStop(1, 'transparent');

        ctx2.fillStyle = gradient2;
        ctx2.beginPath();   // 起始一条路径,或重置当前路径
        ctx2.arc(half, half, half, 0, Math.PI * 2);
        ctx2.fill();    // 填充当前绘制(路径)

        // canvas 绘制的大小
        $(window).resize(function () {
            w = canvas.width = window.innerWidth;
            h = canvas.height = window.innerHeight;
            canvas.width(w).height(h);

        });
        
        // 最大 和 最小星星的 随机位置
        function random(min, max) {
            if (arguments.length < 2) {
                max = min;
                min = 0;
            }

            if (min > max) {
                var hold = max;
                max = min;
                min = hold;
            }

            return Math.floor(Math.random() * (max - min + 1)) + min;
        }

        // 最大和最小的 星星 
        function maxOrbit(x, y) {
            var max = Math.max(x, y),
                diameter = Math.round(Math.sqrt(max * max + max * max));
            return diameter / 2;
        }

        
        var Star = function () {
            this.orbitRadius = random(maxOrbit(w, h));
            this.radius = random(60, this.orbitRadius) / 12;
            this.orbitX = w / 2;
            this.orbitY = h / 2;
            this.timePassed = random(0, maxStars);
            this.speed = random(this.orbitRadius) / 900000;
            this.alpha = random(2, 10) / 10;

            count++;
            stars[count] = this;
        }

        Star.prototype.draw = function () {
            var x = Math.sin(this.timePassed) * this.orbitRadius + this.orbitX,
                y = Math.cos(this.timePassed) * this.orbitRadius + this.orbitY,
                twinkle = random(10);

            if (twinkle === 1 && this.alpha > 0) {
                this.alpha -= 0.05;
            } else if (twinkle === 2 && this.alpha < 1) {
                this.alpha += 0.05;
            }

            ctx.globalAlpha = this.alpha;
            ctx.drawImage(canvas2, x - this.radius / 2, y - this.radius / 2, this.radius, this.radius);
            this.timePassed += this.speed;
        }

        for (var i = 0; i < maxStars; i++) {
            new Star();
        }

        function animation() {
            ctx.globalCompositeOperation = 'source-over';
            ctx.globalAlpha = 0.8;
            ctx.fillStyle = 'hsla(' + hue + ', 64%, 6%, 1)';
            ctx.fillRect(0, 0, w, h)

            ctx.globalCompositeOperation = 'lighter';
            for (var i = 1, l = stars.length; i < l; i++) {
                stars[i].draw();
            };

            window.requestAnimationFrame(animation);
        }

        animation();
    </script>

</body>

</html>
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