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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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Insecure Resource

The resource you are linking to is using the 'http' protocol, which may not work when the browser is using https.

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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Any URL's added here will be added as <link>s in order, and before the CSS in the editor. You can use the CSS from another Pen by using it's URL and the proper URL extention.

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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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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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Packages

Add Packages

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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About Packages

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 

              
                <main>
  <div class="buttons">
    <button onclick="addDot()">Add</button>
    <button onclick="removeDot()">Remove</button>
  </div>
  <ol id="list">
    <li>Vestibulum ac diam sit amet quam vehicula elementum sed sit amet dui.</li>
    <li>Donec sollicitudin molestie malesuada.</li>
    <li>Quisque velit nisi, pretium ut lacinia in, elementum id enim.</li>
    <li>Vivamus magna justo, lacinia eget consectetur sed, convallis at tellus.</li>
    <li>Pellentesque in ipsum id orci porta dapibus.</li>
    <li>Donec sollicitudin molestie malesuada.</li>
  </ol>
</main>
!

CSS 

              
                li:nth-last-child(-n + 3) {
  color: red;
}

li:first-child {
  color: blue;
}

li:nth-last-child(-n + 3):first-child {
  font-weight: bold;
  color: black;
}
!

JS 

              
                function addDot() {
  var item = document.createElement("li");
  item.innerHTML = randomLorem();
  var dots = document.getElementById("list");
  
  dots.appendChild(item);
}

function removeDot() {
  var count = document.getElementById('list').children.length;
  var lastItem = document.getElementById('list').children[count - 1].remove();
}

function randomLorem() {
  var lorem = [ 
    'Pellentesque in ipsum id orci porta dapibus. Vivamus suscipit tortor eget felis porttitor volutpat.',
    'Cras ultricies ligula sed magna dictum porta. Pellentesque in ipsum id orci porta dapibus.',
    'Pellentesque in ipsum id orci porta dapibus. Vivamus suscipit tortor eget felis porttitor volutpat.',
    'Quisque velit nisi, pretium ut lacinia in, elementum id enim. Mauris blandit aliquet elit, eget tincidunt nibh pulvinar a.',
    'Curabitur arcu erat, accumsan id imperdiet et, porttitor at sem. Curabitur arcu erat, accumsan id imperdiet et, porttitor at sem.',
    'Vestibulum ac diam sit amet quam vehicula elementum sed sit amet dui. Quisque velit nisi, pretium ut lacinia in, elementum id enim.',
    'Proin eget tortor risus. Vivamus magna justo, lacinia eget consectetur sed, convallis at tellus.',
    'Vestibulum ac diam sit amet quam vehicula elementum sed sit amet dui. Cras ultricies ligula sed magna dictum porta.',
    'Cras ultricies ligula sed magna dictum porta. Sed porttitor lectus nibh.',
  ];
  return lorem[Math.floor(Math.random() * lorem.length)];
}
!
999px

Console