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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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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.

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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 

              
                <p>Preserved as it was just so that you can see what a pain it was to emulate a conic gradient with a linear one, but please keep in mind that this is not recommended code today anymore. Use <a href="https://codepen.io/thebabydino/pen/qBELQwz" target="_blank">the 2020 version instead</a>.</p>
<div class='rainbow-wheel'></div>
!

CSS 

              
                html { overflow: hidden }
body { background: gainsboro; } 
.rainbow-wheel {
  position: relative;
  margin: 1em auto;
  border: solid 2em white;
  width: 16em; height: 16em;
  border-radius: 50%;
  background: 
    linear-gradient(36deg, #272b66 42.34%, transparent 42.34%) 0 0,
    linear-gradient(72deg, #2d559f 75.48%, transparent 75.48%) 0 0,
    linear-gradient(-36deg, #9ac147 42.34%, transparent 42.34%) 100% 0,
    linear-gradient(-72deg, #639b47 75.48%, transparent 75.48%) 100% 0, 
    linear-gradient(36deg, transparent 57.66%, #e1e23b 57.66%) 100% 100%,
    linear-gradient(72deg, transparent 24.52%, #f7941e 24.52%) 100% 100%,
    linear-gradient(-36deg, transparent 57.66%, #662a6c 57.66%) 0 100%,
    linear-gradient(-72deg, transparent 24.52%, #9a1d34 24.52%) 0 100%, 
    #43a1cd linear-gradient(#ba3e2e, #ba3e2e) 50% 100%;
  background-repeat: no-repeat;
  background-size: 50% 50%;
  transition: 1s
}

.rainbow-wheel::after {
  position: absolute;
  top: 50%; left: 50%;
  margin: -2.8em;
  width: 5.6em; height: 5.6em;
  border-radius: 50%;
  /* add slash at end of line to see central part *
  opacity: .15; /**/
  background: white;
  content: ''
}

.rainbow-wheel:hover { transform: rotate(360deg) }
!

JS 

              
                /*
detailed explanation about the maths of how to compute angles and percentages for the gradients  http://hugogiraudel.com/blog/css-gradients
*/
!
999px

Console