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# HTML小实验：用代码分析HTML标准
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你好，我是winter。
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前面的课程中，我们已经讲解了大部分的HTML标签。
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然而，为了突出重点，我们还是会忽略一些标签类型。比如表单类标签和表格类标签，我认为只有少数前端工程师用过，比如我在整个手机淘宝的工作生涯中，一次表格类标签都没有用到，表单类则只用过input，也只有几次。
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那么，剩下的标签我们怎么样去了解它们呢？当然是查阅HTML标准。
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由于阅读标准有一定门槛，需要了解一些机制，这节课，我为你设计了一个小实验，用JavaScript代码去抽取标准中我们需要的信息。
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## HTML标准
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我们采用WHATWG的living standard标准，我们先来看看标准是如何描述一个标签的，这里我们看到，有下面这些内容。
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```
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Categories:
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Flow content.
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Phrasing content.
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Embedded content.
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If the element has a controls attribute: Interactive content.
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Palpable content.
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Contexts in which this element can be used:
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Where embedded content is expected.
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Content model:
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If the element has a src attribute: zero or more track elements, then transparent, but with no media element descendants.
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If the element does not have a src attribute: zero or more source elements, then zero or more track elements, then transparent, but with no media element descendants.
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Tag omission in text/html:
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Neither tag is omissible.
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Content attributes:
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Global attributes
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src — Address of the resource
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crossorigin — How the element handles crossorigin requests
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poster — Poster frame to show prior to video playback
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preload — Hints how much buffering the media resource will likely need
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autoplay — Hint that the media resource can be started automatically when the page is loaded
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playsinline — Encourage the user agent to display video content within the element's playback area
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loop — Whether to loop the media resource
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muted — Whether to mute the media resource by default
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controls — Show user agent controls
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width — Horizontal dimension
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height — Vertical dimension
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DOM interface:
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[Exposed=Window, HTMLConstructor]
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interface HTMLVideoElement : HTMLMediaElement {
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[CEReactions] attribute unsigned long width;
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[CEReactions] attribute unsigned long height;
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readonly attribute unsigned long videoWidth;
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readonly attribute unsigned long videoHeight;
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[CEReactions] attribute USVString poster;
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[CEReactions] attribute boolean playsInline;
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};
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```
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我们看到，这里的描述分为6个部分，有下面这些内容。
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* Categories：标签所属的分类。
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* Contexts in which this element can be used：标签能够用在哪里。
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* Content model：标签的内容模型。
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* Tag omission in text/html：标签是否可以省略。
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* Content attributes：内容属性。
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* DOM interface：用WebIDL定义的元素类型接口。
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这一节课，我们关注一下Categories、Contexts in which this element can be used、Content model这几个部分。我会带你从标准中抓取数据，做一个小工具，用来检查X标签是否能放入Y标签内。
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## 代码角度分析HTML标准
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HTML标准描述用词非常的严谨，这给我们抓取数据带来了巨大的方便，首先，我们打开单页面版HTML标准：
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* [https://html.spec.whatwg.org/](https://html.spec.whatwg.org/)
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在这个页面上，我们执行一下以下代码：
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```
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Array.prototype.map.call(document.querySelectorAll(".element"), e=>e.innerText);
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```
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这样我们就得到了所有元素的定义了，现在有107个元素。
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不过，比较尴尬的是，这些文本中并不包含元素名，我们只好从id属性中获取，最后代码类似这样：
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```JavaScript
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var elementDefinations = Array.prototype.map.call(document.querySelectorAll(".element"), e => ({
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text:e.innerText,
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name:e.childNodes[0].childNodes[0].id.match(/the\-([\s\S]+)\-element:/)?RegExp.$1:null}));
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```
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接下来我们用代码理解一下这些文本。首先我们来分析一下这些文本，它分成了6个部分，而且顺序非常固定，这样，我们可以用JavaScript的正则表达式匹配来拆分六个字段。
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我们这个小实验的目标是计算元素之间的包含关系，因此，我们先关心一下categories和contentModel两个字段。
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```JavaScript
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for(let defination of elementDefinations) {
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console.log(defination.name + ":")
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let categories = defination.text.match(/Categories:\n([\s\S]+)\nContexts in which this element can be used:/)[1].split("\n");
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for(let category of categories) {
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console.log(category);
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}
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/*
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let contentModel = defination.text.match(/Content model:\n([\s\S]+)\nTag omission in text\/html:/)[1].split("\n");
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for(let line of contentModel)
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console.log(line);
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*/
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}
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```
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接下来我们来处理category。
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首先category的写法中，最基本的就是直接描述了category的句子，我们把这些不带任何条件的category先保存起来，然后打印出来其它的描述看看：
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```JavaScript
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for(let defination of elementDefinations) {
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//console.log(defination.name + ":")
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let categories = defination.text.match(/Categories:\n([\s\S]+)\nContexts in which this element can be used:/)[1].split("\n");
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defination.categories = [];
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for(let category of categories) {
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if(category.match(/^([^ ]+) content./))
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defination.categories.push(RegExp.$1);
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else
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console.log(category)
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}
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/*
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let contentModel = defination.text.match(/Content model:\n([\s\S]+)\nTag omission in text\/html:/)[1].split("\n");
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for(let line of contentModel)
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console.log(line);
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*/
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}
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```
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这里我们要处理的第一个逻辑是带if的情况。
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然后我们来看看剩下的情况：
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```
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None.
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Sectioning root.
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None.
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Sectioning root.
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None.
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Form-associated element.
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Listed and submittable form-associated element.
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None.
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Sectioning root.
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None.
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If the type attribute is not in the Hidden state: Listed, labelable, submittable, resettable, and autocapitalize-inheriting form-associated element.
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If the type attribute is in the Hidden state: Listed, submittable, resettable, and autocapitalize-inheriting form-associated element.
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Listed, labelable, submittable, and autocapitalize-inheriting form-associated element.
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Listed, labelable, submittable, resettable, and autocapitalize-inheriting form-associated element.
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None.
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Listed, labelable, submittable, resettable, and autocapitalize-inheriting form-associated element.
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Listed, labelable, resettable, and autocapitalize-inheriting form-associated element.
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Labelable element.
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Sectioning root.
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Listed and autocapitalize-inheriting form-associated element.
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None.
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Sectioning root.
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None.
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Sectioning root.
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Script-supporting element.
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```
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这里出现了几个概念：
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* None
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* Sectioning root
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* Form-associated element
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* Labelable element
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* Script-supporting element
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如果我们要真正完美地实现元素分类，就必须要在代码中加入正则表达式来解析这些规则，这里作为今天的课后问题，留给你自己完成。
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接下来我们看看Content Model，我们照例先处理掉最简单点的部分，就是带分类的内容模型：
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```JavaScript
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for(let defination of elementDefinations) {
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//console.log(defination.name + ":")
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let categories = defination.text.match(/Categories:\n([\s\S]+)\nContexts in which this element can be used:/)[1].split("\n");
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defination.contentModel = [];
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let contentModel = defination.text.match(/Content model:\n([\s\S]+)\nTag omission in text\/html:/)[1].split("\n");
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for(let line of contentModel)
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if(line.match(/^([^ ]+) content./))
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defination.contentModel.push(RegExp.$1);
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else
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console.log(line)
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}
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```
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好了，我们照例看看剩下了什么：
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```
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A head element followed by a body element.
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If the document is an iframe srcdoc document or if title information is available from a higher-level protocol: Zero or more elements of metadata content, of which no more than one is a title element and no more than one is a base element.
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Otherwise: One or more elements of metadata content, of which exactly one is a title element and no more than one is a base element.
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Text that is not inter-element whitespace.
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Nothing.
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Text that gives a conformant style sheet.
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One or more h1, h2, h3, h4, h5, h6 elements, optionally intermixed with script-supporting elements.
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Nothing.
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Zero or more li and script-supporting elements.
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Either: Zero or more groups each consisting of one or more dt elements followed by one or more dd elements, optionally intermixed with script-supporting elements.
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Or: One or more div elements, optionally intermixed with script-supporting elements.
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Either: one figcaption element followed by flow content.
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Or: flow content followed by one figcaption element.
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Or: flow content.
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If the element is a child of a dl element: one or more dt elements followed by one or more dd elements, optionally intermixed with script-supporting elements.
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If the element is not a child of a dl element: flow content.
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Transparent, but there must be no interactive content or a element descendants.
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See prose.
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Text.
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If the element has a datetime attribute: Phrasing content.
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Otherwise: Text, but must match requirements described in prose below.
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Nothing.
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Transparent.
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Zero or more source elements, followed by one img element, optionally intermixed with script-supporting elements.
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Nothing.
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Zero or more param elements, then, transparent.
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Nothing.
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If the element has a src attribute: zero or more track elements, then transparent, but with no media element descendants.
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If the element does not have a src attribute: zero or more source elements, then zero or more track elements, then transparent, but with no media element descendants.
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If the element has a src attribute: zero or more track elements, then transparent, but with no media element descendants.
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If the element does not have a src attribute: zero or more source elements, then zero or more track elements, then transparent, but with no media element descendants.
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Nothing.
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Transparent.
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Nothing.
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In this order: optionally a caption element, followed by zero or more colgroup elements, followed optionally by a thead element, followed by either zero or more tbody elements or one or more tr elements, followed optionally by a tfoot element, optionally intermixed with one or more script-supporting elements.
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If the span attribute is present: Nothing.
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If the span attribute is absent: Zero or more col and template elements.
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Nothing.
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Zero or more tr and script-supporting elements.
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Zero or more td, th, and script-supporting elements.
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Nothing.
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Zero or more option, optgroup, and script-supporting elements.
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Either: phrasing content.
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Or: Zero or more option and script-supporting elements.
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Zero or more option and script-supporting elements.
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If the element has a label attribute and a value attribute: Nothing.
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If the element has a label attribute but no value attribute: Text.
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If the element has no label attribute and is not a child of a datalist element: Text that is not inter-element whitespace.
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If the element has no label attribute and is a child of a datalist element: Text.
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Text.
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Optionally a legend element, followed by flow content.
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One summary element followed by flow content.
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Either: phrasing content.
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Or: one element of heading content.
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If there is no src attribute, depends on the value of the type attribute, but must match script content restrictions.
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If there is a src attribute, the element must be either empty or contain only script documentation that also matches script content restrictions.
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When scripting is disabled, in a head element: in any order, zero or more link elements, zero or more style elements, and zero or more meta elements.
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When scripting is disabled, not in a head element: transparent, but there must be no noscript element descendants.
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Otherwise: text that conforms to the requirements given in the prose.
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Nothing (for clarification, see example).
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Transparent
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Transparent, but with no interactive content descendants except for a elements, img elements with usemap attributes, button elements, input elements whose type attribute are in the Checkbox or Radio Button states, input elements that are buttons, select elements with a multiple attribute or a display size greater than 1, and elements that would not be interactive content except for having the tabindex attribute specified.
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```
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这有点复杂，我们还是把它做一些分类，首先我们过滤掉带If的情况、Text和Transparent。
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```JavaScript
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for(let defination of elementDefinations) {
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//console.log(defination.name + ":")
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let categories = defination.text.match(/Categories:\n([\s\S]+)\nContexts in which this element can be used:/)[1].split("\n");
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defination.contentModel = [];
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let contentModel = defination.text.match(/Content model:\n([\s\S]+)\nTag omission in text\/html:/)[1].split("\n");
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for(let line of contentModel)
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if(line.match(/([^ ]+) content./))
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defination.contentModel.push(RegExp.$1);
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else if(line.match(/Nothing.|Transparent./));
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else if(line.match(/^Text[\s\S]*.$/));
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else
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console.log(line)
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}
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```
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这时候我们再来执行看看：
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```
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A head element followed by a body element.
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One or more h1, h2, h3, h4, h5, h6 elements, optionally intermixed with script-supporting elements.
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Zero or more li and script-supporting elements.
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Either: Zero or more groups each consisting of one or more dt elements followed by one or more dd elements, optionally intermixed with script-supporting elements.
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Or: One or more div elements, optionally intermixed with script-supporting elements.
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If the element is a child of a dl element: one or more dt elements followed by one or more dd elements, optionally intermixed with script-supporting elements.
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See prose.
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Otherwise: Text, but must match requirements described in prose below.
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Zero or more source elements, followed by one img element, optionally intermixed with script-supporting elements.
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Zero or more param elements, then, transparent.
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If the element has a src attribute: zero or more track elements, then transparent, but with no media element descendants.
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If the element does not have a src attribute: zero or more source elements, then zero or more track elements, then transparent, but with no media element descendants.
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If the element has a src attribute: zero or more track elements, then transparent, but with no media element descendants.
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If the element does not have a src attribute: zero or more source elements, then zero or more track elements, then transparent, but with no media element descendants.
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In this order: optionally a caption element, followed by zero or more colgroup elements, followed optionally by a thead element, followed by either zero or more tbody elements or one or more tr elements, followed optionally by a tfoot element, optionally intermixed with one or more script-supporting elements.
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If the span attribute is absent: Zero or more col and template elements.
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Zero or more tr and script-supporting elements.
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Zero or more td, th, and script-supporting elements.
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Zero or more option, optgroup, and script-supporting elements.
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Or: Zero or more option and script-supporting elements.
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Zero or more option and script-supporting elements.
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If the element has a label attribute but no value attribute: Text.
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If the element has no label attribute and is not a child of a datalist element: Text that is not inter-element whitespace.
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If the element has no label attribute and is a child of a datalist element: Text.
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When scripting is disabled, in a head element: in any order, zero or more link elements, zero or more style elements, and zero or more meta elements.
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When scripting is disabled, not in a head element: transparent, but there must be no noscript element descendants.
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Otherwise: text that conforms to the requirements given in the prose.
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```
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这下剩余的就少多了，我们可以看到，基本上剩下的都是直接描述可用的元素了，如果你愿意，还可以用代码进一步解析，不过如果是我的话，会选择手工把它们写成JSON了，毕竟只有三十多行文本。
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好了，有了contentModel和category，我们要检查某一元素是否可以作为另一元素的子元素，就可以判断一下两边是否匹配啦，首先，我们要做个索引：
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```JavaScript
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var dictionary = Object.create(null);
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for(let defination of elementDefinations) {
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dictionary[defination.name] = defination;
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}
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```
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然后我们编写一下我们的check函数：
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```JavaScript
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function check(parent, child) {
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for(let category of child.categories)
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if(parent.contentModel.categories.contains(category))
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return true;
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if(parent.contentModel.names.contains(child.name))
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return true;
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return false;
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}
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```
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## 总结
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这一节课，我们完成了一个小实验：利用工具分析Web标准文本，来获得元素的信息。
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通过这个实验，我希望能够传递一种思路，代码能够帮助我们从Web标准中挖掘出来很多想要的信息，编写代码的过程，也是更深入理解标准的契机。
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我们前面的课程中把元素分成了几类来讲解，但是这些分类只能大概地覆盖所有的标签，我设置课程的目标也是讲解标签背后的知识，而非每一种标签的细节。具体每一种标签的属性和细节，可以留给大家自己去整理。
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这一节课的产出，则是“绝对完整的标签列表”，也是我学习和阅读标准的小技巧，通过代码我们可以从不同的侧面分析标准的内容，挖掘需要注意的点，这是一种非常好的学习方法。
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