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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

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

Any URLs added here will be added as <link>s in order, and before the CSS in the editor. You can use the CSS from another Pen by using its URL and the proper URL extension.

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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.

You can also link to another Pen here (use the .css URL Extension) and we'll pull the CSS from that Pen and include it. If it's using a matching preprocessor, use the appropriate URL Extension and we'll combine the code before preprocessing, so you can use the linked Pen as a true dependency.

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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

Any URL's added here will be added as <script>s in order, and run before the JavaScript in the editor. You can use the URL of any other Pen and it will include the JavaScript from that Pen.

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About External Resources

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 

              
                <fieldset>
  <label for="mode">Writing Mode:</label>
  
  <select id="mode" name="mode">
    <option value="default">Default</option>
    <option value="ltr">LTR</option>
    <option value="rtl">RTL</option>
    <option value="vertical-rl">Vertical RL</option>
  </select>
</fieldset>

<div id="content">
  <img class="start" src="https://picsum.photos/id/83/300" alt="The edge of the forest" />
  <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Donec sagittis tincidunt luctus. Aliquam non sem quis nunc pellentesque cursus. Mauris accumsan lorem at mi gravida sollicitudin. Vestibulum pharetra eleifend risus in ultricies. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Morbi elementum varius dui, a efficitur libero gravida sit amet. Nunc aliquam pellentesque nisi eget imperdiet. Nam vitae venenatis justo. Vestibulum rhoncus sodales justo vel efficitur. Sed pellentesque tempor felis nec luctus. Nam hendrerit feugiat pharetra. Sed nec euismod ex. Aenean a velit non quam sodales mollis at nec mauris. Vestibulum vulputate est nibh, vulputate elementum ligula pretium ac. Aliquam sed massa dignissim, dictum arcu eu, maximus metus.</p>
  <img class="end" src="https://picsum.photos/id/48/300" alt="A MacBook on a desk" />
  <p>Vivamus nisl elit, sollicitudin in pulvinar nec, feugiat ut justo. Sed diam quam, laoreet vel pretium ut, rutrum ullamcorper mauris. Nunc quis fringilla enim. Integer mattis libero a nulla egestas, non feugiat sem euismod. Vivamus eget neque lacus. Orci varius natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Suspendisse sagittis viverra nibh, et ultrices justo laoreet vel. Curabitur eu libero nec neque euismod egestas nec a mi. Curabitur posuere, arcu non bibendum ullamcorper, turpis ex sodales nunc, eu suscipit sapien tortor at urna. Vivamus cursus enim et lobortis aliquam. Praesent mattis id nisi at condimentum. Nam accumsan tristique ipsum nec imperdiet. Donec pharetra nisl ut enim placerat, ut dignissim est laoreet. Cras vel orci augue.</p>
  <p>Cras malesuada ultricies turpis sed pulvinar. Praesent hendrerit purus id viverra dignissim. Mauris laoreet ut sapien ac interdum. Sed in pharetra ipsum. Integer a aliquam purus. Class aptent taciti sociosqu ad litora torquent per conubia nostra, per inceptos himenaeos. Phasellus consequat porttitor arcu, quis ullamcorper lorem efficitur quis. Fusce lacus velit, finibus id quam id, pretium varius diam. Integer euismod, metus sed mattis convallis, ex lectus rutrum eros, vitae volutpat eros odio non ligula. Sed nec nulla nec massa malesuada vulputate. Sed vel massa vel risus viverra mollis. Duis eu metus quis ex gravida placerat vitae id lectus. Ut eget nulla sit amet nisl condimentum consequat ut vitae massa. Curabitur sit amet ex tristique, varius dolor at, congue eros.</p>
  <p>Nulla urna felis, lacinia eget eleifend vitae, dapibus in leo. Phasellus euismod molestie ligula, at dapibus orci. Sed nec fermentum risus. Fusce eget tellus justo. Etiam purus purus, tincidunt vitae accumsan nec, pellentesque ut nulla. Cras iaculis lacus ut elit eleifend efficitur. Quisque ligula dui, bibendum vestibulum tellus id, varius maximus augue. Suspendisse potenti.</p>
  <p>Cras sit amet malesuada nibh, ac lacinia ante. In blandit nisl urna, quis vehicula urna viverra eu. Mauris laoreet sapien eget ultricies semper. Phasellus facilisis turpis ultrices nulla suscipit lobortis. Mauris lorem elit, ullamcorper sit amet iaculis lacinia, egestas quis lacus. Nam consectetur erat eu eros vehicula venenatis. Aenean pharetra, libero non aliquet suscipit, dui nisi sodales diam, in consequat mauris mi vel mauris. Donec quis mi quis diam fermentum pellentesque. Aenean congue lacus a odio hendrerit, id facilisis nulla finibus. Ut consectetur, erat ac pharetra dictum, mi leo viverra tellus, sed fringilla eros nisi nec urna.</p>
</div>
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CSS 

              
                img {
  max-width: 40%;
}

img.start {
  margin-inline-end: 1rem;
  margin-block-end: 1rem;
  float: inline-start;
  clear: inline-start;
}

img.end {
  margin-inline-start: 1rem;
  margin-bottom: 1rem;
  float: inline-end;
  clear: inline-end;
}

// BOILERPLATE
* {
  box-sizing: border-box;
}

p {
  margin: 0 0 1.5em;
}

fieldset {
  margin: 1rem auto 4rem;
  max-width: 30em;
  display: flex;
  border: 0;
}

label {
  margin-inline-end: 1rem;
  flex: 0 0 auto;
}

select {
  width: 100%;
}

[id="content"] {
  margin-block: 1rem;
  margin-inline: auto;
  padding: 1rem;
  max-inline-size: 30em;
  
  &[style*="mode"] {
    max-block-size: 100vw;
    overflow-y: scroll;
  }
}
!

JS 

              
                const mode = document.getElementById('mode')
const content = document.getElementById('content')

mode.addEventListener('change', e => {
  switch (e.target.value) {
    case 'default':
    case 'ltr':
    case 'rtl':
      content.setAttribute('dir', e.target.value)
      content.removeAttribute('style')
      break
    case 'vertical-rl':
      content.setAttribute('style', `writing-mode:${e.target.value}`)
      content.removeAttribute('dir')
      break
    default:
      break
  }
})
!
999px

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