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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.

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Babel includes JSX processing.

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Packages

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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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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.

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HTML 

              
                <!-- Emmet Code: 
doc>header>h1{Website Title}+nav>a{Link $}*3^^hr+aside#sidebar>h2{Sidebar Title}+p>Lorem^^main#blog>article.post(h1{Post Title $}+div.post-body(p*2>Lorem))*3
-->
<html>

<head>
  <meta charset="UTF-8">
  <title>Document</title>
</head>

<body>
  <header>
    <h1>Website Title</h1>
    <nav>
      <a href="">Link 1</a></li>
      <a href="">Link 2</a></li>
      <a href="">Link 3</a></li>
    </nav>
  </header>
  <hr>
  <aside id="sidebar">
    <h2>Sidebar Title</h2>
    <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Molestiae deleniti tempore aliquam repudiandae est accusantium iste error! Facilis, sunt soluta totam eum! Laudantium pariatur minus ipsam quibusdam, quod saepe iure?</p>
  </aside>
  <main id="blog">
    <article class="post">
      <h1>Post Title 1</h1>
      <div class="post-body">
        <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Cum tempora, excepturi, laboriosam eligendi eveniet similique quod, saepe veniam id ducimus expedita vitae! Eius mollitia, earum molestias vero, numquam vel maxime.</p>
        <p>Molestias, tenetur voluptate fugit, rerum optio id corporis eum, inventore asperiores deleniti aliquam qui voluptates, ipsam debitis reprehenderit! Molestiae atque possimus adipisci, itaque deserunt non obcaecati autem ipsam numquam commodi!</p>
      </div>
    </article>
    <article class="post">
      <h1>Post Title 2</h1>
      <div class="post-body">
        <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Porro, qui iste quisquam officiis obcaecati perferendis nam a expedita molestias ex dolores id laudantium, eius assumenda commodi suscipit, nisi quam. Nesciunt.</p>
        <p>Pariatur, voluptatem dolor, quae aut eaque tempore mollitia quis in odit quaerat explicabo neque, dignissimos ullam iste tenetur commodi reiciendis numquam, necessitatibus! Suscipit recusandae id repellat, veritatis qui voluptate nobis.</p>
      </div>
    </article>
    <article class="post">
      <h1>Post Title 3</h1>
      <div class="post-body">
        <p>Lorem ipsum dolor sit amet, consectetur adipisicing elit. Numquam eaque tenetur necessitatibus consequuntur, sed earum totam in laudantium nihil ab provident quas pariatur aliquam dolorem fugit. Odio ratione, aperiam dicta.</p>
        <p>Quasi ipsum alias quis, maxime vero, eligendi possimus ex cum incidunt fuga neque dolorum ad atque accusantium beatae architecto earum quaerat, soluta culpa quibusdam. Voluptas ullam maxime quos recusandae pariatur.</p>
      </div>
    </article>
  </main>
</body>

</html>
!

CSS 

              
                body {
  line-height: 1.5;
}

aside h2, aside p {
  padding: 0 5%;
  margin-top: 30px;
}
aside h2,
header h1, 
nav {
  text-align: center;
}
header h1 { font-size: 250%; }
/* Helps with accessibility. */
nav {
  font-size: 0;
  margin-top: -20px;
}
nav a {
  font-size: 18px;
  background: #ddd;
  padding: 5px;
  margin: 0 5px;
}
main,
header {
  max-width: 500px;
  margin: 0 auto;
}

/* For screens smaller than 600 pixels wide. */
@media only screen and (max-width: 599.9px) {
  aside {
    display: none;
  }
  main,
  header {
    margin: 0 40px;
  }
}

/* For screens larger than 600 pixels wide. */
@media only screen and (min-width: 600px) {
  aside {
    position: fixed;
    width: 230px;
    height: 100%;
    top: 0;
    left: 0;
    background: #eee;
  }
  main,
  header {
    max-width: 500px;
    padding-left: 240px;
    margin: 0 auto;
  }
}

/* Demonstrating the debugging hack. */
/* * {
  background: #000!important;
  color: #0f0!important;
  outline: solid #f00 1px!important;
} */

/* Demonstrating the box model hack. */
html {
  box-sizing: border-box;
}
*,
*:before,
*:after {
}
!

JS
!
999px

Console