1. 为什么css需要放在头部？
2. js为什么要放在body后面？
3. 图片的加载和渲染会阻塞页面DOM构建吗？
4. dom解析完才出现页面吗？
5. 首屏时间根据什么来判定？

1.处理 HTML 标记并构建 DOM 树。
2.处理 CSS 标记并构建 CSSOM 树。
3.将 DOM 与 CSSOM 合并成一个渲染树。
4.根据渲染树来布局，以计算每个节点的几何信息。
5.将各个节点绘制到屏幕上。

从 DOM 树的根节点开始遍历每个可见节点。

某些节点不可见（例如脚本标记、元标记等），因为它们不会体现在渲染输出中，所以会被忽略。
某些节点通过 CSS 隐藏，因此在渲染树中也会被忽略，例如，上例中的 span 节点---不会出现在渲染树中，---因为有一个显式规则在该节点上设置了“display: none”属性。
对于每个可见节点，为其找到适配的 CSSOM 规则并应用它们。

发射可见节点，连同其内容和计算的样式。

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<div id="div1">
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<div class="a">
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<div class="b">
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...
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</div>
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<div class="c">
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<div class="d">
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...
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</div>
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<div class="e">
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...
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</div>
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</div>
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</div>
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<div class="f">
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<div class="c">
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<div class="d">
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...
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</div>
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</div>
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</div>
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</div>

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#div1 .c .d {}
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.f .c .d {}
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.a .c .e {}
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#div1 .f {}
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.c .d{}

解析HTML【遇到标签加载图片】 —> 构建DOM树
加载样式 —> 解析样式【遇到背景图片链接不加载】 —> 构建样式规则树
加载javascript —> 执行javascript代码
把DOM树和样式规则树匹配构建渲染树【遍历DOM树时加载对应样式规则上的背景图片】
计算元素位置进行布局
绘制【开始渲染图片】

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#div p.class {
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color: red;
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}
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.class {
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color: red;
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}

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(function(){
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window.gConfig = window.gConfig || {};
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window.gConfig.isMobile = /iPhone|iPad|iPod|Android/i.test(navigator.userAgent);
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var hClassName;
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if(window.gConfig.isMobile){
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hClassName = ' phone';
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document.write('<link rel="stylesheet" href="https://res.hc-cdn.com/cpage-pep-discount-area-v6/2.0.24/m/index.css" />');
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document.write('<link rel="preload" href="//res.hc-cdn.com/cpage-pep-discount-area-v6/2.0.24/m/index.js" crossorigin="anonymous" as="script" />');
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}else{
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hClassName = ' pc';
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document.write('<link rel="stylesheet" href="https://res.hc-cdn.com/cpage-pep-discount-area-v6/2.0.24/pc/index.css" />');
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document.write('<link rel="preload" href="//res.hc-cdn.com/cpage-pep-discount-area-v6/2.0.24/pc/index.js" crossorigin="anonymous" as="script" />');
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}
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var root = document.documentElement;
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root.className += hClassName ;
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})();

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document.addEventListener("DOMContentLoaded", function() {
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let lazyImages = [].slice.call(document.querySelectorAll("img.lazy"));
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let active = false;
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const lazyLoad = function() {
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if (active === false) {
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active = true;
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setTimeout(function() {
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lazyImages.forEach(function(lazyImage) {
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if ((lazyImage.getBoundingClientRect().top <= window.innerHeight && lazyImage.getBoundingClientRect().bottom >= 0) && getComputedStyle(lazyImage).display !== "none") {
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lazyImage.src = lazyImage.dataset.src;
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lazyImage.srcset = lazyImage.dataset.srcset;
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lazyImage.classList.remove("lazy");
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lazyImages = lazyImages.filter(function(image) {
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return image !== lazyImage;
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});
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if (lazyImages.length === 0) {
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document.removeEventListener("scroll", lazyLoad);
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window.removeEventListener("resize", lazyLoad);
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window.removeEventListener("orientationchange", lazyLoad);
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}
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}
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});
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active = false;
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}, 200);
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}
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};
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document.addEventListener("scroll", lazyLoad);
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window.addEventListener("resize", lazyLoad);
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window.addEventListener("orientationchange", lazyLoad);
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});

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$.ajax({
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url: url, async: false, timeout: 10000,
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success: function (data) {
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container.innerHTML = data;
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var appendHtml = $('<div class="footer-wrapper">' + container.querySelector('#footer').innerHTML + '</div>');
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var tempHtml = '<div style="display:none;">' + '<script type="text/html" id="header-lazyload-html-drop" class="header-lazyload-html" data-holder="#holder-drop">' + appendHtml.find('#header-lazyload-html-drop').html() + '<\/script><script type="text/html" id="header-lazyload-html-mbnav" class="header-lazyload-html" data-holder="#holder-mbnav">' + appendHtml.find('#header-lazyload-html-mbnav').html() + '<\/script></div>';
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$('#footer').append(tempHtml);
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feloader.onLoad(function () {
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feloader.use('@cloud/common-resource/header', function () {
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});
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$('#footer').css('display', 'block');
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});
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},
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error: function (XMLHttpRequest, textStatus, errorThrown) {
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console.log(XMLHttpRequest.status, XMLHttpRequest.readyState, textStatus);
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},
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});

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<div class="list">
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<img class="lazyload" data-src="http://www.placehold.it/375x200/eee/444/1" src="占位图片URL" />
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<img class="lazyload" data-src="http://www.placehold.it/375x200/eee/444/2" src="占位图片URL" />
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<img class="lazyload" data-src="http://www.placehold.it/375x200/eee/444/3" src="占位图片URL" />
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<div class="lazyload" data-src="http://www.placehold.it/375x200/eee/444/3"></div>
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...
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</div>

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/**
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* load image
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* @param {HTMLElement} el - the image element
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* @private
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*/
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_load(el) {
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let source = el.getAttribute(ATTR_IMAGE_URL);
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if (source) {
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let processor = this._config.processor;
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if (processor) {
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source = processor(source, el);
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}
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el.addEventListener('load', () => {
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el.classList.remove(CLASSNAME);
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});
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// 判断是否是什么元素
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if (el.tagName === 'IMG') {
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el.src = source;
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} else {
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// 判断source是不是一个类名，如果是类名的话，则加到class里面去
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if (/^[A-Za-z0-9_-]+$/.test(source)) {
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el.classList.add(source);
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} else {
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let styles = el.getAttribute('style') || '';
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styles += `;background-image: url(${source});`;
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el.setAttribute('style', styles);
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el.style.backgroundImage = source; // = `background-image: url(${source});`;
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}
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}
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el.removeAttribute(ATTR_IMAGE_URL);
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}
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}

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<div class="nov-c6-cards j-content">
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</div>

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public render(){
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this.$el.find('.j-content').html(new Xtemplate(tpl).render(mockData))
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...
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}

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import './common/js/AGrid';
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import './common/js/jquery.base64';
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import './common/js/lang-tips';
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import './common/js/setLocaleCookie';
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import './common/js/pepDialog';

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feloader.onLoad(function () {
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feloader.use([
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'@cloud/link-to/index',
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'@cloud/common-resource/uba',
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'@cloud/common-resource/footer',
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'@cloud/common-resource/header',
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'@cloud/common-resource/common',
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'@cloud/common-resource/prompt.css',
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'@cloud/common-resource/footer.css',
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]);
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});

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<link rel="dns-prefetch" href="//res.hc-cdn.com">
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<link rel="dns-prefetch" href="//res-static1.huaweicloud.com">
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<link rel="dns-prefetch" href="//res-static2.huaweicloud.com">
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<link rel="dns-prefetch" href="//res-static3.huaweicloud.com">

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<link rel="preload" href="//res.hc-cdn.com/cnpm-feloader/1.0.6/feloader.js" as="script"/>
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<link rel="preload" href="//polyfill.alicdn.com/polyfill.min.js?features=default,es6" as="script"/>
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<link rel="preload" href="https://res-static3.huaweicloud.com/content/dam/cloudbu-site/archive/commons/3rdlib/jquery/jquery-1.12.4.min.js" as="script"/>
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<link rel="preload" href="//res.hc-cdn.com/cnpm-wpk-reporter/1.0.6/wpk-performance.js" as="script"/>
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<link rel="preload" href="//res.hc-cdn.com/cpage-pep-2019nov-promotion/1.1.15/components/activity-banner/images/banner_mb.jpg" as="image" media="(max-width: 767px)">