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HTML

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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).

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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.

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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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HTML 

              
                <div class="wrapper">  
  <main>
    <div class="content">
      <h2>What is flexbox?</h2>
      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nullam finibus sit amet nisl accumsan volutpat. Pellentesque hendrerit nisl erat, at blandit justo tincidunt sed. Nulla et purus sem. Morbi et hendrerit leo, non commodo velit. Nulla facilisi. Duis et dolor ac tellus maximus elementum. Nulla facilisi. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos.</p>

      <p>Maecenas tincidunt mauris eu est aliquam, sed mattis est faucibus. Integer lectus augue, fringilla eu orci a, facilisis vehicula lorem. Donec quis elementum tellus. Nullam tempus bibendum ultricies. Nunc a pulvinar nisl. Morbi id leo vitae tellus maximus fermentum eget ac enim. Phasellus bibendum viverra justo efficitur eleifend. Cras condimentum sodales sollicitudin. Donec vel arcu convallis, dignissim neque quis, condimentum leo. Maecenas at condimentum diam. Nunc ornare ante eget faucibus malesuada. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus elementum elementum dictum. Praesent dapibus quam eget tellus condimentum, laoreet suscipit tortor ultricies.<p>
      
      <img src="https://cdn.css-tricks.com/wp-content/uploads/2011/08/flexbox.png">
      
      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nullam finibus sit amet nisl accumsan volutpat. Pellentesque hendrerit nisl erat, at blandit justo tincidunt sed. Nulla et purus sem. Morbi et hendrerit leo, non commodo velit. Nulla facilisi. Duis et dolor ac tellus maximus elementum. Nulla facilisi. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos.</p>

      <p>Maecenas tincidunt mauris eu est aliquam, sed mattis est faucibus. Integer lectus augue, fringilla eu orci a, facilisis vehicula lorem. Donec quis elementum tellus. Nullam tempus bibendum ultricies. Nunc a pulvinar nisl. Morbi id leo vitae tellus maximus fermentum eget ac enim. Phasellus bibendum viverra justo efficitur eleifend. Cras condimentum sodales sollicitudin. Donec vel arcu convallis, dignissim neque quis, condimentum leo. Maecenas at condimentum diam. Nunc ornare ante eget faucibus malesuada. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus elementum elementum dictum. Praesent dapibus quam eget tellus condimentum, laoreet suscipit tortor ultricies.<p>
      <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nullam finibus sit amet nisl accumsan volutpat. Pellentesque hendrerit nisl erat, at blandit justo tincidunt sed. Nulla et purus sem. Morbi et hendrerit leo, non commodo velit. Nulla facilisi. Duis et dolor ac tellus maximus elementum. Nulla facilisi. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos.</p>

      <p>Maecenas tincidunt mauris eu est aliquam, sed mattis est faucibus. Integer lectus augue, fringilla eu orci a, facilisis vehicula lorem. Donec quis elementum tellus. Nullam tempus bibendum ultricies. Nunc a pulvinar nisl. Morbi id leo vitae tellus maximus fermentum eget ac enim. Phasellus bibendum viverra justo efficitur eleifend. Cras condimentum sodales sollicitudin. Donec vel arcu convallis, dignissim neque quis, condimentum leo. Maecenas at condimentum diam. Nunc ornare ante eget faucibus malesuada. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Vivamus elementum elementum dictum. Praesent dapibus quam eget tellus condimentum, laoreet suscipit tortor ultricies.<p>
    </div>
    <aside>
      <ul>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
        <li>Lorem Ipsum Dolor Sit Amet</li>
      </ul>
    </aside>  
  </main>
</div>
!

CSS 

              
                *
  margin: 0
  padding: 0
  box-sizing: border-box
html, body
  height: 100%
  font 15px sans-serif
  color #222

.wrapper
  height: 100%
  width: 100%
  display: flex
  flex-direction: column
  background: white

header
  flex: 0 0 50px
  padding: 1rem
  background: #BADA55
  
main
  flex 1 1 auto
  display: flex
  flex-wrap wrap

.content, aside
  padding: 1rem
  max-height: 100%
  overflow-y: scroll

.content
  flex 1 1 80%
  width: 80%
  background: white
  p
    margin-bottom: 1rem
  img
    margin: 1rem
    max-width: 100%

aside
  flex 1 1 auto
  min-width: 150px

  ul
    list-style: none
    -webkit-padding-start: 0px;
  li
    display: block
    padding: 1rem
    background: #EEE
    border-bottom: 1px solid #CCC
    transition: background .5s ease
  li:hover
    background: tomato
    color: black
!

JS
!
999px

Console