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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.

!
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).

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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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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 

              
                <div class="controller">
  <input id="widthInput" type="range" min="100" max="2000" value="800">
  <output id="widthOutput">800</output>
</div>
<main id="mainSection">
  <h1>Hello World</h1>
  <p>
    <span>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Cumque ratione, fugiat voluptatibus delectus sint repudiandae. Dolore quaerat sunt exercitationem dolorum tempora voluptates tenetur, omnis quis blanditiis sed, accusamus labore pariatur?</span>
    <span>Consectetur unde mollitia provident, rem accusamus blanditiis a labore atque maxime, totam aut! Praesentium veniam minima ex sapiente, natus at blanditiis provident, deleniti vel illum facere nulla similique odit? Iure.</span>
    <span>Autem deleniti ipsum quibusdam at numquam. Fugiat reprehenderit iure nulla repudiandae quod recusandae voluptas iusto voluptatum exercitationem itaque! Quia esse aut saepe hic modi, magni consectetur culpa quae corporis minima?</span>
  </p>
</main>
!

CSS 

              
                body {
  margin: 0;
}

main {
  margin-inline: auto;
  max-width: 800px;
}

p {
  text-align: justify
}

.controller {
  align-items: center;
  display: flex;
  gap: 0.5rem;
}
!

JS 

              
                let value = widthInput.value;

main();

function main() {
  widthInput.oninput = () => {
    value = widthInput.value;
    render();
  }
  
  render();
}

function render() {
  widthInput.value = value;
  widthOutput.textContent = `${value}px`;
  mainSection.style.maxWidth = `${value}px`;
}
!
999px

Console