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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

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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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About External Resources

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. For instance, CoffeeScript can help prevent easy-to-make mistakes and offer a cleaner syntax and Babel can bring ECMAScript 6 features to browsers that only support ECMAScript 5.

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Babel is required to process package imports. If you need a different preprocessor remove all packages first.

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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Behavior

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HTML 

              
                // by twitter.com/dbox

body.debug
  section
    article      
      hgroup
        h1 debug()
        
      p  Toggle this on a parent element. It will add a border to its children, grandchildren, etc so you can see what's up - great for precise layout tweaks.  It will also add flags if you made mistakes like put in inline styles, forgot an alt on an image, left the alt blank, etc.
      ul
        li Level 4
          span Level 5
        li Level 4
          span(style="") Level 5 w/ Inline style flag
        li Level 4
      h3 Images
      ul
        li
          p no alt tag:
          img(src='http://dogr.io/doge.png' width="150px" height="150px")         
        li
          p empty alt tag:
          img(src='http://dogr.io/doge.png' width="150px" height="150px" alt="")        
        li
          p all good:
          img(src='http://dogr.io/doge.png' width="150px" height="150px" alt="Alt tags for images")  
     
      h3 Links
      ul
        li
          a empty a tag    
        li
          a(href="#") I link to #
        li
          a(href="http://www.dbox.us" title="Check out my site.") I'm cool
!

CSS 

              
                // Mixin: Debug
//
// Debugging tool - adds a border to the current element, its children, 
// grandchildren, etc so you can see what's up – great for precise layout  
// tweaks.  It will also add flags if you made mistakes like put in inline 
// styles, forgot an alt on an image, left the alt blank, etc. Not to be used 
// in production, obviously.

debug()
  border 1px solid MediumPurple
  > *
    border 1px solid DarkKhaki
  > * > *
    border 1px solid MediumTurquoise
  > * > * > *
    border 1px solid LightSlateGray
  > * > * > * > *
    border 1px solid LightBlue
  > * > * > * > * > *
    border 1px solid MediumSlateBlue
  > * > * > * > * > * > *
    border 1px solid DeepSkyBlue
  > * > * > * > * > * > * > *
    border 1px solid DarkSeaGreen

  img
    border: 1px solid #f00
    -webkit-filter: hue-rotate(330deg) contrast(.75) saturate(6)
    filter: hue-rotate(330deg) contrast(.75) saturate(6)

  img[alt]
    border: 1px solid LimeGreen
    -webkit-filter: none
    filter: none

  img[alt=""]
    border: 1px solid gold
    -webkit-filter: contrast(1) saturate(2)
    filter: contrast(1) saturate(2)

  a
    background: rgba(#f00, .1)
    border: 1px solid #f00

  a[title]
    background: none
    border-color: LimeGreen

  style
    border: block

  [style], style, [class=""], [id=""], a[href="#"]
    background: rgba(yellow, .2)
    border: 1px solid gold

// call it
.debug
  debug()










// Demo styles only - Disregard

body
  font-family Helvetica, Arial, sans
  font-size 16px
  line-height 1.4em
  padding 2% 8%
  margin 0
  
*
  box-sizing border-box

 h1
  font-size 4.4vw
  font-weight normal
  line-height 1em
  margin 0

article
  width 60%
  margin 0 20%
  padding 4%

ul
  list-style-type none
  padding 3%
  margin 0
  overflow hidden

  > li
    display inline-block
    float left
    width calc(100% / 3)
    padding 2%

    a
      float right

    &:last-child
      margin-right 0

code
  background LightSlateGray 
  color white
!

JS
!
999px

Console