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HTML

About HTML Preprocessors

HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

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Adding Classes

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.

About the <head>

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. If you need things in the <head> of the document, put that code here.

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Insecure Resource

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CSS

About CSS Preprocessors

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

About Vendor Prefixing

To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.

We offer two popular choices: Autoprefixer (which processes your CSS server-side) and -prefix-free (which applies prefixes via a script, client-side).

Add External Stylesheets/Pens

Any URL's added here will be added as <link>s in order, and before the CSS in the editor. If you link to another Pen, it will include the CSS from that Pen. If the preprocessor matches, it will attempt to combine them before processing.

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You can apply CSS to your Pen from any stylesheet on the web. Just put a URL to it here and we'll apply it, in the order you have them, before the CSS in the Pen itself.

If the stylesheet you link to has the file extension of a preprocessor, we'll attempt to process it before applying.

You can also link to another Pen here, and we'll pull the CSS from that Pen and include it. If it's using a matching preprocessor, we'll combine the code before preprocessing, so you can use the linked Pen as a true dependency.

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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

Add External Scripts/Pens

Any URL's added here will be added as <script>s in order, and run before the JavaScript in the editor. You can use the URL of any other Pen and it will include the JavaScript from that Pen.

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About External Resources

You can apply a script from anywhere on the web to your Pen. Just put a URL to it here and we'll add it, in the order you have them, before the JavaScript in the Pen itself.

If the script you link to has the file extension of a preprocessor, we'll attempt to process it before applying.

You can also link to another Pen here, and we'll pull the JavaScript from that Pen and include it. If it's using a matching preprocessor, we'll combine the code before preprocessing, so you can use the linked Pen as a true dependency.

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Packages

Add Packages

Search for and use JavaScript packages from npm here. By selecting a package, an import statement will be added to the top of the JavaScript editor for this package.

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About Packages

Using packages here is powered by Skypack, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ES6 import usage.

All packages are different, so refer to their docs for how they work.

If you're using React / ReactDOM, make sure to turn on Babel for the JSX processing.

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HTML 

              
                section 下面是多列展示区域
p 样式里规定是3列, 但内容总数为偶数4, 则列数为2
div.container
  div 1
  div 2
  div 3
  div 4
  
p 下面添加一个内容, 则列数变3列, 因为内容总数为5, 是奇数 

button#add 点我添加5

section 但是看下面, 偶数一样可以三列, 又是怎么回事? 因为有一个子元素设置了
  code margin: 1px 0;

p 只要设置了上下边距中的一个, 就可以达到这样的效果, 设置左右边距是没有效果的

p 设置border也能达到同样的效果, 但padding则不行

div.container
  div.margin 1
  div 2
  div 3
  div 4
  
section 下面看看跨列对顺序的影响

p 这里设置的列数为2 数字3是跨列的
p 在Chrome55下, 排列顺序由从上到下, 变成从左到右了, Firefox则照旧. 这个顺序是依赖具体浏览器的实现

div.col-count-two
  div 1
  div 2
  div.cross-col 3
  div 4
  div 5
!

CSS 

              
                .container
  -webkit-column-count 3
  -moz-column-count 3
  column-count 3
  // column-width 30vw
  // column-gap 1px
  div
    // 加border也会变3列
    // border 1px dotted

.margin
  //有了这句话, 内容为偶数也就能变成3列了
  margin 1px 0
 
.col-count-two
  -webkit-column-count 2
  -moz-column-count 2
  column-count 2
  
.cross-col
  -webkit-column-span all
  -moz-column-span all
  column-span all

section
  margin-bottom 10px
  &::before
    content ''
    display block
    border-top 1px solid
    margin 10px 0
!

JS 

              
                window.onload = function(){
  document.getElementById('add').onclick = function() {
    
   var ele = document.createElement('div')
   
    ele.textContent = 5
    document.querySelector('.container').appendChild(ele)
    
    this.onclick = null
  }
}
!
999px

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