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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>

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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).

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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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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.

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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.

All packages are different, so refer to their docs for how they work.

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HTML 

              
                <p>
  Big spoilers below! Hover to reveal...
</p>
<p class="spoiler">
  Turns out he was dead the whole time. That guy is totally his father and the princess is his sister. They're all allergic to water and everything happens for a reason. He's the chosen one now, but he wasn't the first. What he thought was an alien planet was actually Earth. They turn the humans against each other to make the invasion easier. What you thought were flashbacks actually take place in the future. The simulations were real.
</p>
<p>
  End spoilers. Now you know everything.
</p>
!

CSS 

              
                @import url(https://fonts.googleapis.com/css?family=Arimo|Fjalla+One);

*, *::before, *::after {
  box-sizing: border-box;
}

body {
  padding: 0 10%;
  font: 24px/1.5 'Arimo', sans-serif;
  color: #55657a;
  background-color: #fff;
}

p {
  text-align: justify;
}

.spoiler {
  position: relative;
  color: transparent;
  text-shadow: 0 0 20px #55657a;
  cursor: help;
}

.spoiler::before, .spoiler::after {
  display: block;
  position: absolute;
  line-height: 1;
  font-family: 'Fjalla One', sans-serif;
  color: #872e27;
  text-align: center;
  text-shadow: none;
}

.spoiler::before {
  top: 36px;
  left: 0;
  right: 0;
  font-size: 72px;
  content: 'SPOILER ALERT';
}

.spoiler::after {
  opacity: 0;
  top: 84px;
  left: calc(50% - 36px);
  width: 60px;
  font-size: 60px;
  line-height: 72px;
  text-align: center;
  clip: rect(0, 72px, 72px, 0);
  content: '3 2 1';  
}

.spoiler:hover {
  color: #55657a;
  text-shadow: 0 0 0 transparent;
  transition: all 1s;
  transition-delay: 4s;
}

.spoiler:hover::before {
  animation: countdown-before 5s forwards;
}

.spoiler:hover::after {
  animation: countdown-after 5s forwards;
}

@keyframes countdown-before {
  25% {
    transform: translateY(-24px);
  }
  75% {
    opacity: 1;
  }
  100% {
  	opacity: 0;
    transform: translateY(-24px);
  }
}

@keyframes countdown-after {
  25% {
    opacity: 1;
    clip: rect(0, 72px, 72px, 0);
    transform: translateY(0);
    animation-timing-function: steps(1, start);
  }
  50% {
    clip: rect(72px, 72px, 144px, 0);
    transform: translateY(-72px);
    animation-timing-function: steps(1, start);
  }
  75% {
    opacity: 1;
    clip: rect(144px, 72px, 216px, 0);
    transform: translateY(-144px);
  }
  100% {
    opacity: 0;
    clip: rect(144px, 72px, 216px, 0);
    transform: translateY(-144px);
  }
}
!

JS
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