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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

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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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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 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 

              
                <div class="container">
<h1>Amet dicta dolar.</h1>

<p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Illum eum perspiciatis blanditiis ipsum numquam perferendis. Blanditiis at amet aspernatur eius ipsam! Officia earum aliquid reprehenderit magni fuga praesentium! Iste, est.</p>
<p>Repudiandae aut doloremque ea blanditiis praesentium adipisci asperiores eius molestias reiciendis consequuntur sit ipsam quis aperiam nostrum tenetur rem quibusdam inventore amet, alias facere dolor! Tempora cupiditate laborum natus molestiae?</p>
  <h2>Recusandae sed perferendis autem deserunt</h2>
  <p>Eum, natus reiciendis, suscipit fugit tempore ex cum ut? Quod nam accusamus perferendis qui distinctio consectetur repellat harum sunt tempore? Obcaecati explicabo numquam ex neque.</p>
<p>Illum dolore, nobis officia ipsa unde, culpa exercitationem cumque laboriosam vero autem dicta, quia amet totam repellat minus recusandae. Dolorum repellendus culpa illo eaque est totam sit eum atque optio.</p>
<p>Nemo consectetur provident ab, doloremque obcaecati nam officiis enim nihil rerum, quis, dolor beatae? Consequuntur consequatur dolores facilis, hic eius enim deleniti, ullam asperiores minus quos at, ad natus illo!</p>
<p>Assumenda deserunt laborum magni quo ratione sapiente placeat necessitatibus quidem. Maiores cupiditate tempore, repudiandae similique nisi molestiae culpa molestias reiciendis quas animi ad obcaecati blanditiis sapiente quia praesentium a esse!</p>
  <h3>Nobis quaerat fugiat laborum</h3>
<p>A dicta molestias ex explicabo in iure ducimus culpa asperiores eaque earum. Quia provident aperiam deleniti corrupti fugit. Nobis quaerat fugiat laborum iusto. Dolor eum, necessitatibus libero distinctio eveniet temporibus.</p>
<p>Dolorum sequi harum distinctio nemo laboriosam, esse, veritatis similique enim possimus eligendi eum. Voluptatem minima quos est vero. Ducimus tempore numquam eveniet, accusamus aperiam magnam reiciendis dolore quia reprehenderit dolor!</p>
  <h3>Facilis voluptates rem at eum</h3>
  <p>In esse, dolore earum error veniam accusamus quo veritatis consequatur nulla laboriosam mollitia provident ipsam sit praesentium eveniet sapiente. Deleniti quos laudantium repellendus beatae quam.</h3>
<p>Recusandae laborum animi eum qui quos ut, aut consectetur minima, atque nam ea illum iusto suscipit asperiores unde non molestias rerum? Illum, quo quam ipsa illo sapiente laudantium rerum exercitationem!</p>
</div>
!

CSS 

              
                :root {
  font-size: 100%;
  box-sizing: border-box;
  --hue: 210;
}

*,
*::before,
*::after {
  box-sizing: inherit;
}

body {
  margin: 0;
  font: 3vmax/160% Segoe UI, sans-serif;
}

h1, h2, h3 { 
  line-height: 1.1;
  margin: 1em 0 1.5rem;
}

h1 {
  font-size: 8vmax;
}

h2 {
  font-size: 6vmax;
}

h3 {
  font-size: 4vmax;
}

.container {
  padding: 10vmin;
}
!

JS
!
999px

Console