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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

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).

Add External Stylesheets/Pens

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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.

Add External Scripts/Pens

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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 

              
                <a class="fancy" href="#">Our fancy link</a><br>
<a class="buggy" href="#">Buggy, but almost good</a><br>
<a class="just-border" href="#">Just a border</a><br>
<a class="old-school" href="#">Old school regular</a><br>

<div class="row">
  <p class="column">Cat ipsum dolor sit amet, annoy the old grumpy cat. <a class="fancy" href="#">Notice how well it works with multi-line links! Chew the plant run in circles. Cats go for world domination.</a> I like frogs and 0 gravity yet eat half my food and ask for more.</p>
   <p class="column">Just <a class="buggy" href="#">in case you are wondering, this is Cat Ipsum. Does the underline appear buggy?</a> Have my breakfast spaghetti yarn step on your keyboard while you're gaming and then turn in a circle. Destroy house in 5 seconds.
  </p>
</div>
!

CSS 

              
                a.fancy, a.fancy:visited {
    text-decoration: none;
    background-image:
      linear-gradient(
          transparent 2px,
          #2ed392 2px,
          #2ed392 4px,
          transparent 4px
      ),
      linear-gradient(
          transparent 2px,
          #d8dce9 2px,
          #d8dce9 4px,
          transparent 4px
      );
    background-size: 0% 6px, 100% 6px;
    background-position: 0 bottom, 0 bottom;
    transition: background-size 0.3s ease-in-out;
    background-repeat: no-repeat;
    padding-bottom: 4px;
    border-bottom: 6px solid transparent;
}
a.fancy:hover {
    background-size: 100% 6px;
}
@supports (-ms-ime-align:auto) {
    a.fancy, a.fancy:visited {
        background-image:
            linear-gradient(#2ed392, #2ed392),
            linear-gradient(#d8dce9, #d8dce9);
        background-size: 0% 2px, 100% 2px;
        padding-bottom: 2px;
    }
    a.fancy:hover {
        background-size: 100% 2px;
    }
}

/* Only border as underline, animated*/
a.just-border, a.just-border:visited {
    text-decoration: none;
    border-bottom: 2px solid #d8dce9;
    transition: border-color 0.3s;
}
a.just-border:hover {
    border-bottom: 2px solid #2ed392;
}

/* Regular normal old school link */
a.old-school, a.old-school:visited {
    text-decoration: none;
}
a.old-school:hover {
    text-decoration: underline;
}

/* Buggy approach*/
a.buggy, a.buggy:visited {
    text-decoration: none;
    position: relative;
    border-bottom: 2px solid #d8dce9;
}
a.buggy:before {
    content: "";
    height: 100%;
    position: absolute;
    border-bottom: 2px solid #2ed392;
    margin-top: 2px;
    left: 0;
    right: 100%;
    transition: all 0.3s;
}
a.buggy:hover:before {
    right: 0;
}

/* Cosmetics of the example only */
a:not(.old-school) {
    color: #2d305f;
}
a, p {
    font-size: 21px;
    line-height: 1.4;
}
p {
    color: #333346;
    max-width: 300px;
}

.row {
    display: flex;
}
.column {
    flex: 50%;
    margin-right: 10px;
    margin-bottom: 0;
}
@media only screen and (max-width: 640px) {
    .row {
      display: block;
    }
}
body{
  margin: 20px;
}
!

JS
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999px

Console