万维网的核心语言、标准通用标记语言下的一个应用超文本标记语言(HTML)的第五次重大修改(这是一项推荐标准、外语原文：W3C Recommendation、见本处参考资料原文内容：)。

2014年10月29日，万维网联盟宣布，经过接近8年的艰苦努力，该标准规范终于制定完成。

《妙味课堂HTML5视频教程》 将会介绍超文本标记语言(HTML)的第五次重大修改，即最新版本；它比原来的标准又增加了一些新的标签，实现更多功能，更标准化，更适用于移动互联网。

本视频学习难点在于canvas的用法：

仿知乎背景canvas特效

body{

text-align: center;

background: #F7FAFC;

overflow: hidden;

background: #fff;

}

//定义画布宽高和生成点的个数

var WIDTH = window.innerWidth, HEIGHT = window.innerHeight, POINT = 35;

var canvas = document.getElementById('Mycanvas');

canvas.width = WIDTH,

canvas.height = HEIGHT;

var context = canvas.getContext('2d');

context.strokeStyle = 'rgba(0,0,0,0.2)',

context.strokeWidth = 1,

context.fillStyle = 'rgba(0,0,0,0.1)';

var circleArr = [];

//线条：开始xy坐标，结束xy坐标，线条透明度

function Line (x, y, _x, _y, o) {

this.beginX = x,

this.beginY = y,

this.closeX = _x,

this.closeY = _y,

this.o = o;

}

//点：圆心xy坐标，半径，每帧移动xy的距离

function Circle (x, y, r, moveX, moveY) {

this.x = x,

this.y = y,

this.r = r,

this.moveX = moveX,

this.moveY = moveY;

}

//生成max和min之间的随机数

function num (max, _min) {

var min = arguments[1] || 0;

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

}

// 绘制原点

function drawCricle (cxt, x, y, r, moveX, moveY) {

var circle = new Circle(x, y, r, moveX, moveY)

cxt.beginPath()

cxt.arc(circle.x, circle.y, circle.r, 0, 2*Math.PI)

cxt.closePath()

cxt.fill();

return circle;

}

//绘制线条

function drawLine (cxt, x, y, _x, _y, o) {

var line = new Line(x, y, _x, _y, o)

cxt.beginPath()

cxt.strokeStyle = 'rgba(0,0,0,'+ o +')'

cxt.moveTo(line.beginX, line.beginY)

cxt.lineTo(line.closeX, line.closeY)

cxt.closePath()

cxt.stroke();

}

//每帧绘制

function draw () {

context.clearRect(0,0,canvas.width, canvas.height);

for (var i = 0; i < POINT; i++) {

drawCricle(context, circleArr[i].x, circleArr[i].y, circleArr[i].r);

}

for (var i = 0; i < POINT; i++) {

for (var j = 0; j < POINT; j++) {

if (i + j < POINT) {

var A = Math.abs(circleArr[i+j].x - circleArr[i].x),

B = Math.abs(circleArr[i+j].y - circleArr[i].y);

var lineLength = Math.sqrt(A*A + B*B);

var C = 1/lineLength*7-0.009;

var lineOpacity = C > 0.03 ? 0.03 : C;

if (lineOpacity > 0) {

drawLine(context, circleArr[i].x, circleArr[i].y, circleArr[i+j].x, circleArr[i+j].y, lineOpacity);

}

}

}

}

}

//初始化生成原点

function init () {

circleArr = [];

for (var i = 0; i < POINT; i++) {

circleArr.push(drawCricle(context, num(WIDTH), num(HEIGHT), num(15, 2), num(10, -10)/40, num(10, -10)/40));

}

draw();

}

//调用执行

window.onload = function () {

init();

setInterval(function () {

for (var i = 0; i < POINT; i++) {

var cir = circleArr[i];

cir.x += cir.moveX;

cir.y += cir.moveY;

if (cir.x > WIDTH) cir.x = 0;

else if (cir.x < 0) cir.x = WIDTH;

if (cir.y > HEIGHT) cir.y = 0;

else if (cir.y < 0) cir.y = HEIGHT;

}

draw();

}, 10);

}