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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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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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If enabled, the preview panel updates automatically as you code. If disabled, use the "Run" button to update.

Format on Save

If enabled, your code will be formatted when you actively save your Pen. Note: your code becomes un-folded during formatting.

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HTML Settings

Here you can Sed posuere consectetur est at lobortis. Donec ullamcorper nulla non metus auctor fringilla. Maecenas sed diam eget risus varius blandit sit amet non magna. Donec id elit non mi porta gravida at eget metus. Praesent commodo cursus magna, vel scelerisque nisl consectetur et.

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HTML 

              
                <div class="panel" data-color="white">
    <div>
      <h1>Magic scrolling colours</h1>
      <p>Scroll to animate the background colour of the body as a full height panel becomes visible.</p>
      <p>I have tried to comment the code, particurly the JavaScript, as much as possible. I hope it's clear to understand.</p>
      <p>If you have any questions my twitter is <a href="https://twitter.com/daveredfern">@daveredfern</a>.</p>
  </div>
</div>
<div class="panel" data-color="violet">
  <h2>Violet panel</h2>
</div>
<div class="panel" data-color="indigo">
  <h2>Indigo panel</h2>
</div>
<div class="panel" data-color="blue">
  <h2>Blue panel</h2>
</div>
<div class="panel" data-color="green">
  <h2>Green panel</h2>
</div>
<div class="panel" data-color="yellow">
  <h2>Yellow panel</h2>
</div>
<div class="panel" data-color="orange">
  <h2>Orange panel</h2>
</div>
<div class="panel" data-color="red">
  <h2>Red panel</h2>
</div>
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CSS 

              
                /* Setting fade transition and default settings */
body {
  color: #000;
  background-color: #f4f4f4;
  transition: background-color 1s ease;
}

/* panel styles */
.panel {
  /* min height incase content is higher than window height */
  min-height: 100vh;
  display: flex;
  justify-content: space-around;
  align-items: center;
  font-family: sans-serif;
  /* outline: 10px solid hotpink; */
  /* turn above on to see the edge of panels */
}

/* colours */
.color-violet {
  background-color: #7A4EAB;
}
.color-indigo {
  background-color: #4332CF;
}
.color-blue {
  background-color: #2F8FED;
}
.color-green {
  background-color: #4DCF42;
}
.color-yellow {
  background-color: #FAEB33;
}
.color-orange {
  background-color: #F19031;
}
.color-red {
  background-color: #F2293A;
}

/* styling for demo, can ignore */
body {
  text-align: center;
  font-size: 120%;
  line-height: 1.618;
}
h1, h2 {
  font-size: 3em;
  letter-spacing: -0.05em;
  line-height: 1.1;
}
p {
  max-width: 30em;
  margin-bottom: 1.618em;
}
a {
  color: #4332CF;
}
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JS 

              
                $(window).scroll(function() {
  
  // selectors
  var $window = $(window),
      $body = $('body'),
      $panel = $('.panel');
  
  // Change 33% earlier than scroll position so colour is there when you arrive.
  var scroll = $window.scrollTop() + ($window.height() / 3);
 
  $panel.each(function () {
    var $this = $(this);
    
    // if position is within range of this panel.
    // So position of (position of top of div <= scroll position) && (position of bottom of div > scroll position).
    // Remember we set the scroll to 33% earlier in scroll var.
    if ($this.position().top <= scroll && $this.position().top + $this.height() > scroll) {
          
      // Remove all classes on body with color-
      $body.removeClass(function (index, css) {
        return (css.match (/(^|\s)color-\S+/g) || []).join(' ');
      });
       
      // Add class of currently active div
      $body.addClass('color-' + $(this).data('color'));
    }
  });    
  
}).scroll();
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