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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

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

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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

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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.

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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.

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HTML 

              
                <header>
	<h1>Site name</h1>
</header>
<main>
	<input type="checkbox" id="contents" /><label for="contents">Toggle contents</label>
	<p>Bacon ipsum dolor sit amet turkey veniam shankle, culpa short ribs kevin t-bone occaecat. Et laborum venison nostrud, ut veniam sint kielbasa ullamco pancetta. Qui drumstick tail, bacon leberkas shoulder capicola laborum. Minim ipsum bacon, mollit laboris t-bone pariatur. Ham hock reprehenderit sint beef, sausage pig eiusmod t-bone shankle strip steak.</p>
	<p>Cow enim excepteur, boudin dolore lorem magna fugiat consequat voluptate. Picanha fugiat chicken, cupim aliquip magna filet mignon prosciutto ut nostrud. Kielbasa rump frankfurter sunt corned beef. Andouille in cillum deserunt, rump et picanha landjaeger tongue anim.</p>
	<p>Ad meatball ipsum ground round shank. Quis ipsum meatball exercitation. Laborum swine spare ribs, sunt ball tip magna t-bone venison. Velit doner voluptate non occaecat do ribeye kevin strip steak et. Esse biltong shank ribeye dolor pariatur aute deserunt non est eiusmod pork belly pancetta pork chop. Pork chop id consectetur rump, meatball short loin brisket tail andouille deserunt alcatra irure prosciutto do.</p>
	<p>Dolore reprehenderit ex, meatball doner commodo consectetur ea ribeye. Ad aliqua kevin, chuck excepteur minim et cow esse ham hock landjaeger. Alcatra bresaola dolore tempor do, excepteur in velit flank officia dolore meatloaf corned beef picanha. Eu pancetta brisket eiusmod ipsum aute sausage, culpa rump shoulder excepteur nostrud venison sed pork loin. Tempor proident do magna ground round. Ut venison frankfurter et veniam dolore. Pig pork belly beef ribs kevin, doner exercitation magna esse shankle.</p>
	<p>Flank anim chuck boudin id consectetur bresaola ham pork loin cupim andouille frankfurter. Proident do ball tip nostrud nulla sed, frankfurter ut commodo corned beef ut. Ex aute in, pig deserunt beef ribs turducken pastrami irure ball tip pork belly pork chop sausage id. Chicken sunt nisi tempor sed. In eiusmod non fatback tempor tenderloin pastrami adipisicing cow lorem ut tail jerky cupidatat venison. Jowl consequat commodo pork loin ipsum pork belly prosciutto aute beef. Ball tip shoulder aliqua, fugiat landjaeger kevin pork chop beef ribs leberkas hamburger cillum turkey ut doner culpa.</p>
</main>
<footer>
	<p>© 2015 No rights reserved.</p>
	<p>Made with ♥ by an anonymous pastafarian.</p>
</footer>
!

CSS 

              
                main {
    min-height: calc(100vh - 2.5em - 7em);
    /* Avoid padding/borders screwing up our height: */
    box-sizing: border-box;
}

/* Toggle checkbox to alternate between short/long content */
#contents:checked ~ p { display: none }

/* Basic styling */
body {
	margin: 0;
	font: 100%/1.5 Palatino Linotype, Palatino, serif;
}

h1 { margin: .5em 0 0; }

header { text-align: center; height: 3em; }

main, footer {
	display: block;
	padding: .5em calc(50% - 400px);
}

footer {
	background: linear-gradient(#222, #444);
	color: white;
	height: 6em;
}
!

JS
!
999px

Console