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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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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-.

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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

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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 

              
                <article>
  <h1>Accessible footnotes with CSS</h1>

  <p>Maintaining <a href="#footnotes" aria-describedby="footnote-label" id="footnotes-ref">footnotes</a> manually can be a pain. By using <a href="#css" aria-describedby="footnote-label" id="css-ref">CSS</a> <a href="#css-counters" aria-describedby="footnote-label" id="css-counters-ref">counters</a> to add the numbered references in the text and an ordered list to display the actual footnotes in the footer, it becomes extremely easy.</p>

  <footer>
    <h2 class="visually-hidden" id="footnote-label">Footnotes</h2>
    <ol>
      <li id="footnotes">Footnotes are notes placed at the bottom of a page. They cite references or comment on a designated part of the text above it. <a href="#footnotes-ref" aria-label="Back to content">↩</a></li>
      
      <li id="css">Cascading Style Sheets <a href="#css-ref" aria-label="Back to content">↩</a></li>

      <li id="css-counters">CSS counters are, in essence, variables maintained by CSS whose values may be incremented by CSS rules to track how many times they're used. <a href="#css-counters-ref" aria-label="Back to content">↩</a></li>
    </ol>
  </footer>
</article>
!

CSS 

              
                html {
  box-sizing: border-box;
}

* {
  box-sizing: inherit;
}

body {
  margin: 20px;
  font-size: 125%;
  line-height: 1.4;
  max-width: 600px;
  margin: 0 auto;
}

footer {
  margin-top: 50px;
  border-top: 1px solid silver;
  font-size: 0.8em;
}

footer ol {
  padding-left: 20px;
}

/**
 * Initialiazing a `footnotes` counter on the wrapper
 */
article {
  counter-reset: footnotes;
}

/**
 * Inline footnotes references
 * 1. Increment the counter at each new reference
 * 2. Reset link styles to make it appear like regular text
 */
[aria-describedby="footnote-label"] {
  counter-increment: footnotes; /* 1 */
  text-decoration: none; /* 2 */
  color: inherit; /* 2 */
  cursor: default; /* 2 */
  outline: none; /* 2 */
}

/**
 * Actual numbered references
 * 1. Display the current state of the counter (e.g. `[1]`)
 * 2. Align text as superscript
 * 3. Make the number smaller (since it's superscript)
 * 4. Slightly offset the number from the text
 * 5. Reset link styles on the number to show it's usable
 */
[aria-describedby="footnote-label"]::after {
  content: '[' counter(footnotes) ']'; /* 1 */
  vertical-align: super; /* 2 */
  font-size: 0.5em; /* 3 */
  margin-left: 2px; /* 4 */
  color: blue; /* 5 */
  text-decoration: underline; /* 5 */
  cursor: pointer; /* 5 */
}

/**
 * Resetting the default focused styles on the number
 */
[aria-describedby="footnote-label"]:focus::after {
  outline: thin dotted;
  outline-offset: 2px;
}

[aria-label="Back to content"] {
  font-size: 0.8em;
}

/**
 * Highlight target note
 */
footer :target {
  background: yellow;
}

/**
 * Visually hidden yet accessible content
 */
.visually-hidden {
  position: absolute;
  clip: rect(0 0 0 0);
  visibility: hidden;
  opacity: 0;
}
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JS
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