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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.

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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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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 

              
                <main>
  <div class="wrapper">
    <article class="flow">
      <h1>Next sibling selector - out of control margin</h1>
      <figure class="callout">
        <p>
          Because the CSS applied with the <code>* + *</code> selector is recursive,
          we can get unwanted behaviour like unpredictable space.
        </p>
        <p>
          The semi-transparent red boxes show the margin being applied both where you might expect it to be and where you probably don’t want it to be.</p>
      </figure>
      <div class="top box">
        <p>I have no margin because I am the first child.</p>
        <p>
          I have <code>margin-top</code> because I am a <strong>next sibling</strong>.
        </p>
        <p>
          I have a <strong>bold</strong> <em>then an emphasised bit of text</em>,
          which because I put <code>inline-block</code> on my emphasised text, gives
          us unwanted spacing.
        </p>
      </div>
    </article>
  </div>
</main>
!

CSS 

              
                .top * + * {
  margin-top: 1.5em;
}

em {
  display: inline-block;
}

/* Presentation styles */
.top p {
  border: 1px solid var(--color-primary);
  background: var(--color-light);
  padding: 1em;
}

/* Presentational styles */
.top * + * {
  position: relative;
}

.top * + *::before {
  content: "";
  width: 100%;
  height: 1.5em;
  background: #ff807b;
  position: absolute;
  bottom: 100%;
  left: 0;
  outline: 1px solid #9d0600;
  opacity: 0.5;
}
!

JS
!
999px

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