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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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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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HTML 

              
                <main>
  <h2>CSS Expand/Collapse Section</h2>
  <input id="toggle" type="checkbox" checked>
  <label for="toggle">Hidden Kitten</label>
  <div id="expand">
    <section>
      <p>mew</p>
    </section>
  </div>
  <section>
    <h3>Other content</h3>
    <p>Lorem ipsum dolor sit amet, consectetur adipiscing elit. Maecenas porta non turpis faucibus lobortis. Curabitur non eros rutrum, gravida felis non, luctus velit. Ut commodo congue velit feugiat lobortis. Etiam nec dolor quis nulla bibendum blandit vitae nec enim. Maecenas id dignissim erat. Aenean ac mi nec ante venenatis interdum quis vel lacus.
    </p>
    <p>Aliquam ligula est, aliquet et semper vitae, elementum eget dolor. In ut dui id leo tristique iaculis eget a dui. Vestibulum cursus, dolor sit amet lacinia feugiat, turpis odio auctor nisi, quis pretium dui elit at est. Pellentesque lacus risus, vulputate sed gravida eleifend, accumsan ac ante. Donec accumsan, augue eu congue condimentum, erat magna luctus diam, adipiscing bibendum sem sem non elit.</p>
  </section>
</main>
!

CSS 

              
                @import url(https://fonts.googleapis.com/css?family=Open+Sans:400,700);

body {
  font-family: "Open Sans", Arial;
  background: #CCC;
}
main {
  background: #EEE;
  width: 600px;
  margin: 20px auto;
  padding: 10px 0;
  box-shadow: 0 3px 5px rgba(0,0,0,0.3);
}
h2 {
  text-align: center;
}
p {
  font-size: 13px;
}
input {
  display: none;
  visibility: hidden;
}
label {
  display: block;
  padding: 0.5em;
  text-align: center;
  border-bottom: 1px solid #CCC;
  color: #666;
}
label:hover {
  color: #000;
}
label::before {
  font-family: Consolas, monaco, monospace;
  font-weight: bold;
  font-size: 15px;
  content: "+";
  vertical-align: text-top;
  display: inline-block;
  width: 20px;
  height: 20px;
  margin-right: 3px;
  background: radial-gradient(ellipse at center, #CCC 50%, transparent 50%);
}
#expand {
  height: 0px;
  overflow: hidden;
  transition: height 0.5s;
  background: url(http://placekitten.com/g/600/300);
  color: #FFF;
}
section {
  padding: 0 20px;
}
#toggle:checked ~ #expand {
  height: 250px;
}
#toggle:checked ~ label::before {
  content: "-";
}
!

JS
!
999px

Console