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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.

!
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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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.

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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 esm.sh, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ESM usage.

All packages are different, so refer to their docs for how they work.

If you're using React / ReactDOM, make sure to turn on Babel for the JSX processing.

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HTML 

              
                <h1>Lorem ipsum dolor sit amet</h1>

<p><button type="button" onclick="toggleBodyClass();">Toggle <code>body</code> styles</button>
</p>

<p>
Consectetur adipiscing elit. Nulla sollicitudin suscipit velit, cursus accumsan nisi semper eu. Nam viverra orci eu risus euismod, maximus molestie mauris ullamcorper. Praesent venenatis convallis risus, a viverra arcu ullamcorper id. Vivamus semper posuere nisi quis pulvinar. Mauris malesuada tempus nulla eget pulvinar. Cras fermentum tincidunt orci elementum fringilla. Donec a libero a arcu pellentesque auctor. Duis cursus sit amet erat vel luctus. Curabitur neque est, laoreet lobortis mauris eu, venenatis egestas tortor. Fusce ut diam blandit, convallis turpis ultricies, imperdiet felis. In arcu metus, dignissim vitae velit at, sodales ullamcorper lorem. In hac habitasse platea dictumst. Morbi neque ex, bibendum eget lorem in, laoreet finibus mi.
</p>

<p>
Donec a est non lacus molestie dignissim. Etiam scelerisque rhoncus tempus. Donec consequat lectus velit, efficitur viverra nunc convallis non. Sed tincidunt nisi ac placerat scelerisque. Morbi id turpis et augue placerat tincidunt non eget risus. Vestibulum dictum dolor ullamcorper, consequat ligula facilisis, lobortis dolor. Nunc tincidunt nunc non justo accumsan pretium. Vestibulum placerat libero sem, quis blandit nisi fermentum sed.
</p>

<h2>Phasellus eu tortor tempor</h2>

<p>
Lacinia enim eu, sagittis magna. Proin lectus ipsum, efficitur eu ipsum non, ornare porta justo. Aenean nulla nunc, semper vitae risus non, blandit sodales tortor. In leo erat, pulvinar in dolor sit amet, vulputate aliquam urna. Mauris luctus dignissim erat id ullamcorper. Ut condimentum viverra nisl ac pellentesque. Donec id purus tincidunt, egestas sem sed, efficitur justo. Suspendisse malesuada eu libero vel posuere. Phasellus ut justo pulvinar, blandit ipsum vel, pretium nisl.
</p>

<p>
Nam consectetur dictum justo. Maecenas vitae magna eget nisl consequat dapibus sed eu felis. Etiam tristique maximus pulvinar. Donec laoreet condimentum nisl vitae elementum. Vivamus aliquam varius sem, vitae dapibus est mollis ac. Sed aliquet, justo nec mollis tristique, ligula tortor vulputate metus, nec consequat velit felis ac sapien. Sed scelerisque mattis ligula ac maximus. Donec rhoncus elit ut nisl ultricies tempus. Proin sollicitudin gravida velit sit amet mollis. Curabitur nunc leo, tempor sodales eros at, egestas ullamcorper felis. Donec dapibus turpis sapien, sit amet aliquam nunc fringilla vitae. Maecenas ante metus, aliquet suscipit tortor at, dictum dapibus turpis. Quisque finibus bibendum sollicitudin. Curabitur vitae felis ut felis laoreet tincidunt. Aenean euismod augue at auctor viverra. Nunc efficitur, nibh consectetur efficitur mattis, leo ipsum rhoncus nibh, eget elementum est sapien eget nunc.
</p>

<p>
Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Sed finibus magna a sapien imperdiet euismod. Morbi risus mi, vulputate et ultricies et, facilisis ut tellus. Cras porta blandit ante, ac faucibus risus placerat ut. Proin volutpat, turpis eget scelerisque finibus, sem purus gravida orci, in pharetra lectus justo at mi. Curabitur euismod lectus nec quam posuere commodo. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Duis ac fringilla turpis, at molestie mi. Nam bibendum lobortis lectus, ac hendrerit lorem pulvinar non. Curabitur sit amet lacinia lorem.
</p>
!

CSS 

              
                body {
  font-family: "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto,
    Oxygen-Sans, Ubuntu, Cantarell, "Helvetica Neue", sans-serif;
  line-height: 1.4;
}

body.styled {
  background-color: #eee;
/*   color: #333; */
}
!

JS 

              
                function toggleBodyClass() {
  var theBody = document.querySelector("body");
  if (theBody.classList.contains("styled")) {
    theBody.classList.remove("styled");
  } else {
    theBody.classList.add("styled");
  }
}
!
999px

Console