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HTML小实验：用代码分析HTML标准

HTML小实验：用代码分析HTML标准-极客时间

HTML小实验：用代码分析HTML标准

2019-04-11 winter 来自北京
《重学前端》
课程介绍

讲述：winter

时长07:51大小7.20M

你好，我是 winter。
前面的课程中，我们已经讲解了大部分的 HTML 标签。
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。
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。
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。

HTML 标准

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。
Categories:
Flow content.
Phrasing content.
Embedded content.
If the element has a controls attribute: Interactive content.
Palpable content.
Contexts in which this element can be used:
Where embedded content is expected.
Content model:
If the element has a src attribute: zero or more track elements, then transparent, but with no media element descendants.
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.
Tag omission in text/html:
Neither tag is omissible.
Content attributes:
Global attributes
src — Address of the resource
crossorigin — How the element handles crossorigin requests
poster — Poster frame to show prior to video playback
preload — Hints how much buffering the media resource will likely need
autoplay — Hint that the media resource can be started automatically when the page is loaded
playsinline — Encourage the user agent to display video content within the element's playback area
loop — Whether to loop the media resource
muted — Whether to mute the media resource by default
controls — Show user agent controls
width — Horizontal dimension
height — Vertical dimension
DOM interface:
[Exposed=Window, HTMLConstructor]
interface HTMLVideoElement : HTMLMediaElement {
[CEReactions] attribute unsigned long width;
[CEReactions] attribute unsigned long height;
readonly attribute unsigned long videoWidth;
readonly attribute unsigned long videoHeight;
[CEReactions] attribute USVString poster;
[CEReactions] attribute boolean playsInline;
};
我们看到，这里的描述分为 6 个部分，有下面这些内容。
Categories：标签所属的分类。
Contexts in which this element can be used：标签能够用在哪里。
Content model：标签的内容模型。
Tag omission in text/html：标签是否可以省略。
Content attributes：内容属性。
DOM interface：用 WebIDL 定义的元素类型接口。
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。

代码角度分析 HTML 标准

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：
https://html.spec.whatwg.org/
在这个页面上，我们执行一下以下代码：
Array.prototype.map.call(document.querySelectorAll(".element"), e=>e.innerText);
这样我们就得到了所有元素的定义了，现在有 107 个元素。
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：
var elementDefinations = Array.prototype.map.call(document.querySelectorAll(".element"), e => ({
text:e.innerText,
name:e.childNodes[0].childNodes[0].id.match(/the\-([\s\S]+)\-element:/)?RegExp.$1:null}));
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。
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。
for(let defination of elementDefinations) {
console.log(defination.name + ":")
let categories = defination.text.match(/Categories:\n([\s\S]+)\nContexts in which this element can be used:/)[1].split("\n");
for(let category of categories) {
console.log(category);
}
/*
let contentModel = defination.text.match(/Content model:\n([\s\S]+)\nTag omission in text\/html:/)[1].split("\n");
for(let line of contentModel)
console.log(line);
*/
}
接下来我们来处理 category。
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：
for(let defination of elementDefinations) {
//console.log(defination.name + ":")
let categories = defination.text.match(/Categories:\n([\s\S]+)\nContexts in which this element can be used:/)[1].split("\n");
defination.categories = [];
for(let category of categories) {
if(category.match(/^([^ ]+) content./))
defination.categories.push(RegExp.$1);
else
console.log(category)
}
/*
let contentModel = defination.text.match(/Content model:\n([\s\S]+)\nTag omission in text\/html:/)[1].split("\n");
for(let line of contentModel)
console.log(line);
*/
}
这里我们要处理的第一个逻辑是带 if 的情况。
然后我们来看看剩下的情况：
None.
Sectioning root.
None.
Sectioning root.
None.
Form-associated element.
Listed and submittable form-associated element.
None.
Sectioning root.
None.
If the type attribute is not in the Hidden state: Listed, labelable, submittable, resettable, and autocapitalize-inheriting form-associated element.
If the type attribute is in the Hidden state: Listed, submittable, resettable, and autocapitalize-inheriting form-associated element.
Listed, labelable, submittable, and autocapitalize-inheriting form-associated element.
Listed, labelable, submittable, resettable, and autocapitalize-inheriting form-associated element.
None.
Listed, labelable, submittable, resettable, and autocapitalize-inheriting form-associated element.
Listed, labelable, resettable, and autocapitalize-inheriting form-associated element.
Labelable element.
Sectioning root.
Listed and autocapitalize-inheriting form-associated element.
None.
Sectioning root.
None.
Sectioning root.
Script-supporting element.
这里出现了几个概念：
None
Sectioning root
Form-associated element
Labelable element
Script-supporting element
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。
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：
for(let defination of elementDefinations) {
//console.log(defination.name + ":")
let categories = defination.text.match(/Categories:\n([\s\S]+)\nContexts in which this element can be used:/)[1].split("\n");
defination.contentModel = [];
let contentModel = defination.text.match(/Content model:\n([\s\S]+)\nTag omission in text\/html:/)[1].split("\n");
for(let line of contentModel)
if(line.match(/^([^ ]+) content./))
defination.contentModel.push(RegExp.$1);
else
console.log(line)
}
好了，我们照例看看剩下了什么：
A head element followed by a body element.
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.
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.
Text that is not inter-element whitespace.
Nothing.
Text that gives a conformant style sheet.
One or more h1, h2, h3, h4, h5, h6 elements, optionally intermixed with script-supporting elements.
Nothing.
Zero or more li and script-supporting elements.
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.
Or: One or more div elements, optionally intermixed with script-supporting elements.
Either: one figcaption element followed by flow content.
Or: flow content followed by one figcaption element.
Or: flow content.
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.
If the element is not a child of a dl element: flow content.
Transparent, but there must be no interactive content or a element descendants.
See prose.
Text.
If the element has a datetime attribute: Phrasing content.
Otherwise: Text, but must match requirements described in prose below.
Nothing.
Transparent.
Zero or more source elements, followed by one img element, optionally intermixed with script-supporting elements.
Nothing.
Zero or more param elements, then, transparent.
Nothing.
If the element has a src attribute: zero or more track elements, then transparent, but with no media element descendants.
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.
If the element has a src attribute: zero or more track elements, then transparent, but with no media element descendants.
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.
Nothing.
Transparent.
Nothing.
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.
If the span attribute is present: Nothing.
If the span attribute is absent: Zero or more col and template elements.
Nothing.
Zero or more tr and script-supporting elements.
Zero or more td, th, and script-supporting elements.
Nothing.
Zero or more option, optgroup, and script-supporting elements.
Either: phrasing content.
Or: Zero or more option and script-supporting elements.
Zero or more option and script-supporting elements.
If the element has a label attribute and a value attribute: Nothing.
If the element has a label attribute but no value attribute: Text.
If the element has no label attribute and is not a child of a datalist element: Text that is not inter-element whitespace.
If the element has no label attribute and is a child of a datalist element: Text.
Text.
Optionally a legend element, followed by flow content.
One summary element followed by flow content.
Either: phrasing content.
Or: one element of heading content.
If there is no src attribute, depends on the value of the type attribute, but must match script content restrictions.
If there is a src attribute, the element must be either empty or contain only script documentation that also matches script content restrictions.
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.
When scripting is disabled, not in a head element: transparent, but there must be no noscript element descendants.
Otherwise: text that conforms to the requirements given in the prose.
Nothing (for clarification, see example).
Transparent
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.
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。
for(let defination of elementDefinations) {
//console.log(defination.name + ":")
let categories = defination.text.match(/Categories:\n([\s\S]+)\nContexts in which this element can be used:/)[1].split("\n");
defination.contentModel = [];
let contentModel = defination.text.match(/Content model:\n([\s\S]+)\nTag omission in text\/html:/)[1].split("\n");
for(let line of contentModel)
if(line.match(/([^ ]+) content./))
defination.contentModel.push(RegExp.$1);
else if(line.match(/Nothing.|Transparent./));
else if(line.match(/^Text[\s\S]*.$/));
else
console.log(line)
}
这时候我们再来执行看看：
A head element followed by a body element.
One or more h1, h2, h3, h4, h5, h6 elements, optionally intermixed with script-supporting elements.
Zero or more li and script-supporting elements.
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.
Or: One or more div elements, optionally intermixed with script-supporting elements.
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.
See prose.
Otherwise: Text, but must match requirements described in prose below.
Zero or more source elements, followed by one img element, optionally intermixed with script-supporting elements.
Zero or more param elements, then, transparent.
If the element has a src attribute: zero or more track elements, then transparent, but with no media element descendants.
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.
If the element has a src attribute: zero or more track elements, then transparent, but with no media element descendants.
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.
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.
If the span attribute is absent: Zero or more col and template elements.
Zero or more tr and script-supporting elements.
Zero or more td, th, and script-supporting elements.
Zero or more option, optgroup, and script-supporting elements.
Or: Zero or more option and script-supporting elements.
Zero or more option and script-supporting elements.
If the element has a label attribute but no value attribute: Text.
If the element has no label attribute and is not a child of a datalist element: Text that is not inter-element whitespace.
If the element has no label attribute and is a child of a datalist element: Text.
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.
When scripting is disabled, not in a head element: transparent, but there must be no noscript element descendants.
Otherwise: text that conforms to the requirements given in the prose.
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。
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：
var dictionary = Object.create(null);
for(let defination of elementDefinations) {
dictionary[defination.name] = defination;
}
然后我们编写一下我们的 check 函数：
function check(parent, child) {
for(let category of child.categories)
if(parent.contentModel.categories.contains(category))
return true;
if(parent.contentModel.names.contains(child.name))
return true;
return false;
}

总结

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。
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。
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。
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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精选留言(10)

  • 阿成
    2019-04-14
    这种“通过简单的文本分析，快速提炼出自己感兴趣的部分”的方法是非常值得借鉴的，我平时也会用这种方法去网页中做一些快速的统计和信息筛选。 不过，通过这样的文本分析去完成一个“检查一个元素是否能够放置在另一个元素内部”的小程序还是有点“把问题复杂化”的感觉（尽管这个过程中也可以锻炼一些能力），况且文档是会更新的，指不定有一天那些check分支就hold不住新的case了。 在我看来，如果想知道A元素是否可以放在B元素中，只要把所有元素的categories和contentModel提取出来，筛选出A元素的categories和B元素的contentModel，再去阅读比较就可以了（当然你还要对标准中的一些术语有所了解，所幸的是这些术语都有超链接指向定义，所以还是比较方便的ヾ(≧▽≦*)o）。
    展开
    19
  • 一步
    2019-04-29
    老师 有个疑问： WHATWG 和 W3C 标准以哪个为准，这两个标准有什么区别？是不是相互不认可的
    9
  • 前端男孩
    2020-02-16
    为什么我去网页控制台上Console出不来呢？
    3
  • 会飞的大猫
    2019-04-11
    Winter，刚看完文章，就在淘宝技术节视频看到了你持相机和大家自拍的图片
    1
  • Clors
    2020-09-04
    我提出一个场景，如果vw布局网页，不使用iframe如何做到限制最大大小？
  • Change
    2020-03-07
    本想实践一下这个实验，奈何https://html.spec.whatwg.org/链接打不开是什么情况？
    共 1 条评论
  • 爱学习的大叔
    2019-06-02
    没太看懂，好多语法基于这个页面https://html.spec.whatwg.org/
  • away
    2019-04-30
    @一步 WHATWG 和 W3C 标准若有不同，一般以 WHATWG 为准
  • 嗨海海
    2019-04-12
    学不到，有因果关系，工作实际需要吗？
    共 1 条评论
  • 被雨水过滤的空气
    2019-04-11
    学习了