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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

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.

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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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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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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.

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HTML 

              
                <select class="custom-select">
  <option>
    <svg aria-hidden xmlns="http://www.w3.org/2000/svg" width="1lh" height="1lh" viewBox="0 0 128 128"><path fill="currentColor" d="m9.032 2l10.005 112.093l44.896 12.401l45.02-12.387L118.968 2zm89.126 26.539l-.627 7.172L97.255 39H44.59l1.257 14h50.156l-.336 3.471l-3.233 36.119l-.238 2.27L64 102.609v.002l-.034.018l-28.177-7.423L33.876 74h13.815l.979 10.919L63.957 89H64v-.546l15.355-3.875L80.959 67H33.261l-3.383-38.117L29.549 25h68.939z"/></svg>
    <span>HTML</span>
  </option>
  <option>
    <svg aria-hidden xmlns="http://www.w3.org/2000/svg" width="1lh" height="1lh" viewBox="0 0 32 32"><path fill="currentColor" d="M1.995 1.994h23.52a4.48 4.48 0 0 1 4.48 4.48v19.04a4.48 4.48 0 0 1-4.48 4.48H6.475a4.48 4.48 0 0 1-4.48-4.48Z"/><path fill="Canvas" d="M9.079 24.87v-4.704c0-1.876 1.204-2.884 3.024-2.884c1.792-.028 2.912 1.148 2.856 3.136h-2.072c.056-.756-.28-1.316-.84-1.288c-.7 0-.896.476-.896 1.372v4.088c0 .868.28 1.288.896 1.316c.644 0 .896-.644.84-1.372h2.072c.112 2.044-1.176 3.248-2.996 3.22c-1.764 0-2.884-.98-2.884-2.884m6.636-.336h1.932c.028.896.308 1.456.924 1.456s.84-.364.84-1.204c0-.7-.308-1.092-1.064-1.456l-.728-.336c-1.288-.616-1.82-1.372-1.82-2.884c0-1.68 1.064-2.856 2.8-2.856s2.66 1.204 2.688 3.164h-1.876c0-.812-.168-1.372-.784-1.372c-.56 0-.84.28-.84.98s.252.98.924 1.26l.672.308c1.428.672 2.044 1.54 2.044 3.164c0 1.932-1.092 2.996-2.884 2.996s-2.8-1.232-2.828-3.22m6.328 0h1.96c0 .896.308 1.456.896 1.456s.84-.364.84-1.204c0-.7-.28-1.092-1.064-1.456l-.728-.336c-1.288-.616-1.792-1.372-1.792-2.884c0-1.68 1.036-2.856 2.8-2.856s2.632 1.204 2.688 3.164h-1.876c-.028-.812-.196-1.372-.812-1.372c-.56 0-.812.28-.812.98s.224.98.896 1.26l.7.308c1.4.672 2.016 1.54 2.016 3.164c0 1.932-1.092 2.996-2.884 2.996s-2.8-1.232-2.828-3.22"/></svg>
    <span>CSS</span>
  </option>
  <option>
    <svg aria-hidden xmlns="http://www.w3.org/2000/svg" width="1lh" height="1lh" viewBox="0 0 512 512"><path fill="currentColor" d="M32 32v448h448V32Zm240 348c0 43.61-25.76 64.87-63.05 64.87c-33.68 0-53.23-17.44-63.15-38.49l34.28-20.75c6.61 11.73 11.63 21.65 26.06 21.65c12 0 21.86-5.41 21.86-26.46V240h44Zm99.35 63.87c-39.09 0-64.35-17.64-76.68-42L329 382c9 14.74 20.75 24.56 41.5 24.56c17.44 0 27.57-7.72 27.57-19.75c0-14.43-10.43-19.54-29.68-28l-10.52-4.52c-30.38-12.92-50.52-29.16-50.52-63.45c0-31.57 24.05-54.63 61.64-54.63c26.77 0 46 8.32 59.85 32.68L396 290c-7.22-12.93-15-18-27.06-18c-12.33 0-20.15 7.82-20.15 18c0 12.63 7.82 17.74 25.86 25.56l10.52 4.51c35.79 15.34 55.94 31 55.94 66.16c.01 37.9-29.76 57.64-69.76 57.64"/></svg>
    <span>JavaScript</span>
  </option>
  <option>
    <svg aria-hidden xmlns="http://www.w3.org/2000/svg" width="1lh" height="1lh" viewBox="0 0 32 32"><path fill="currentColor" d="M19.153 2.35v.15a3.2 3.2 0 1 1-6.4 0v-.15H2v27.919h27.919V2.35Z"/><path fill="Canvas" d="M8.485 17.4h1.85l1.265 6.723h.023L13.14 17.4h1.731l1.371 6.81h.027l1.44-6.81h1.815l-2.358 9.885h-1.837l-1.36-6.728h-.036l-1.456 6.728h-1.87Zm13.124 0h2.917l2.9 9.885h-1.911l-.63-2.2h-3.323l-.486 2.2h-1.859Zm1.11 2.437l-.807 3.627h2.512l-.924-3.632Z"/></svg>
    <span>WASM</span>
  </option>
</select>
!

CSS 

              
                @supports (appearance: base-select) {
  .custom-select {
    /* 
      new customizable appearance assignment.
      this changes what can go inside <select> and 
      exposes parts you can target and style
    */
    &, &::picker(select) {
      appearance: base-select;
    }
    
    /* nice layout for the selected state */
    selectedcontent {
      display: flex;
      align-items: center;
      gap: .5ch;
      
      /* hide anything not an svg */
      > :not(svg) {
        display: none;
      }
    }
    
    /* icon at the end with grid */
    option {
      display: grid;
      grid-template-columns: [icon] auto [content] 1fr [mark] auto;
      
      &::checkmark {
        grid-area: 1 / mark;
      }
    }
  }
}













@layer support {
  html {
    block-size: 100%;
    color-scheme: dark light;
  }

  body {
    min-block-size: 100%;
    font-family: system-ui, sans-serif;

    display: flex;
    place-content: center;
    place-items: center;
    gap: 5vmin;
  }
}
!

JS 

              
                document.querySelector('.custom-select').addEventListener('change', event => {
  console.log(event.target.value)
})
!
999px

Console