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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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HTML 

              
                <p>
  Using HTML from <a href="http://www.marcozehe.de/2013/02/02/advanced-aria-tip-1-tabs-in-web-apps/">Advanced ARIA tip #1: Tabs in web apps</a>, slightly tweaked, and my own CSS and JavaScript.
</p>

<ul class="tabList" id="tabs" role="tablist">
  <li role="presentation"><a id="tab1" href="#panel1" onclick="showTab(1);return false;" role="tab" aria-controls="panel1" aria-selected="false">Tab 1</a></li>
  <li role="presentation"><a id="tab2" href="#panel2" onclick="showTab(2);return false;" role="tab" aria-controls="panel2" aria-selected="false">Tab 2</a></li>
  <li role="presentation"><a id="tab3" href="#panel3" onclick="showTab(3);return false;" role="tab" aria-controls="panel3" aria-selected="false">Tab 3</a></li>
</ul>

<div class="tabPanels">
  <div id="panel1" role="tabpanel" aria-labelledby="tab1">
  <p>
    Nulla tincidunt pharetra tortor. In dapibus ultricies arcu. Suspendisse at purus eu est tincidunt feugiat. Praesent <a href="foo">et sapien></a>. Vivamus fermentum, diam vel ornare vestibulum, nibh massa imperdiet lectus, eget tincidunt urna urna nec erat. Curabitur interdum. Nam lorem nunc, posuere quis, suscipit eu, hendrerit vitae, nisi. Etiam hendrerit tincidunt felis.
  </p>
  </div>

  <div id="panel2" role="tabpanel" aria-labelledby="tab2">
  <p>
    Vestibulum id eros eu lorem tincidunt sollicitudin. Suspendisse ligula. Sed nisi magna, elementum at, ultricies in, tincidunt imperdiet, quam. Nulla semper. <a href="foo">Suspendisse potenti</a>. Sed sollicitudin dolor aliquet purus. Aliquam dui. Proin arcu metus, porttitor eget, pulvinar nec, molestie dapibus, ligula.
  </p>
  </div>

  <div id="panel3" role="tabpanel" aria-labelledby="tab3">
    <p>
      Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Aliquam vel erat. <a href="foo">Vestibulum egestas purus ut felis</a>. Cum sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus.
    </p>
  </div>
</div>

<script>
  showTab(1);
</script>
!

CSS 

              
                .html {
  background-color: #ccc;
	/*background-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQCAMAAAAoLQ9TAAAACVBMVEUAAAAAAAAAAACDY+nAAAAAA3RSTlMmDQBzGIDBAAAAG0lEQVR42uXIIQEAAADCMHj/0NdkQMws0HEeAqvwAUGJthrXAAAAAElFTkSuQmCC);*/
  color: #000;
  font-family: Arial, Helvetica, sans-serif;
}

.tabList {
  list-style-type: none;
  padding: 0;
  margin: 0 auto;
}

.tabList a {
  display: block;
  float: left;
  border: 1px solid #000;
  padding: .25em 2em;
  margin: 0 0 -1px .25em;
  border-radius: .5em .5em 0 0;
  background-color: #aaa;
}

.tabList a:link, .tabList a:visited, .tabList a:hover, .tabList a:focus, .tabList a:active {
  text-decoration: none;
  color: #000;
}

.tabList a:hover, .tabList a:focus {
  background-color: #ccc;
}

.tabList a.selected {
  background-color: #fff;
  border-bottom: 1px solid #fff;
}

.tabPanels div {
  clear: left;
  margin: 0 auto;
  padding: 1em 2em;
  border: 1px solid #000;
  border-radius: .25em;
  background-color: #fff;
  display: none;
}

.tabPanels div.selected, div[aria-hidden=false] {
  display: block;
}

.hide, div[aria-hidden=true] {
  display: none;
}
!

JS 

              
                var OpenTab;

function showTab(num) {
  try{
      if(OpenTab!=undefined){
        var OldTabID = document.getElementById('tab'+OpenTab);
        var OldPanelID = document.getElementById('panel'+OpenTab);
        OldTabID.className = '';
        OldPanelID.className = 'hide';
        OldTabID.setAttribute('aria-selected', false);
        OldPanelID.setAttribute('aria-hidden', true);
      }
      var TabID = document.getElementById('tab'+num);
      var PanelID = document.getElementById('panel'+num);
      TabID.className = 'selected';
      PanelID.className = 'selected';
      TabID.setAttribute('aria-selected', true);
      PanelID.setAttribute('aria-hidden', false);
      OpenTab = num;
  }catch(e){}
}

showTab(1);
!
999px

Console