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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

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.

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.

!
Insecure Resource

The resource you are linking to is using the 'http' protocol, which may not work when the browser is using https.

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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If the script you link to has the file extension of a preprocessor, we'll attempt to process it before applying.

You can also link to another Pen here, and we'll pull the JavaScript from that Pen and include it. If it's using a matching preprocessor, we'll combine the code before preprocessing, so you can use the linked Pen as a true dependency.

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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 

              
                <div class='container'>
  <h1>
    Buttons
  </h1>
  <p>
    Random selection of button styles. May change over time as I get alot of stupid ideas. Feel free to follow me on  <a href="https://twitter.com/simonbusborg" target="_blank" class="twitter">twitter</a>
  </p>
  <div class="button-container">
    <div class='button -regular center'>Let's start</div>
    
    <div class='button -dark center'>Touch me</div>
    
    <div class='button -green center'>Just like that</div>
    
    <div class='button -blue center'>And that, oh, yeah</div>
        
    <div class='button -salmon center'>Now that I like</div>
    
    <div class='button -sun center'>God, that's so nice</div>
    
    <div class='button -alge center'>Now lower down</div>
    
    <div class='button -flower center'>Where the figs lie</div>
    
  </div>
</div>
!

CSS 

              
                // colors
:root {
  --color-dark:  #161616;
  --color-ocean:  #416dea;
  --color-grass: #3dd28d;
  --color-snow: #FFFFFF;
  --color-salmon: #F32C52;
  --color-sun: #feee7d;
  --color-alge: #7999a9;
  --color-flower: #353866;
  --color-smoke: #e4e4e4;
  
  --borderRadius: 36px;

  --font-face: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol";
}


// reset
html {
  box-sizing: border-box;
}
*, *:before, *:after {
  box-sizing: inherit;
}



// body styles
body {
  display: flex;

  box-sizing: border-box;
  min-height: 100vh;

  background: whitesmoke;
  font-family: var(--font-face);
  font-size: 16px;
  font-weight: 700;
  line-height: 1;

  justify-content: center;
  align-items: center;
  
  @media (min-width: 992px) {
    border-width: 30px;
  }
}

body.linked {
  
  .button, h1 {
    opacity: .3;
    transition: opacity 300ms linear;
  }
  
  p {
    color: var(--color-smoke);
    transition: color 300ms linear;
  }
}




// text styles
h1 {
  text-align: center;
  
  transition: opacity 300ms linear;

  color: var(--color-dark);

  font-family: var(--font-face);
  font-size: 32px;
  font-weight: 700;
  
  span {
    font-weight: 400;
  }
}

p {
  max-width: 460px;
  margin: 0 auto;

  transition: color 300ms linear;
  text-align: center; 

  color: var(--color-dark);

  font-weight: 400;
  line-height: 1.5;
}

a, a:visited {
  text-decoration: none;

  color: var(--color-ocean);

  font-weight: 500;
  
  &:hover {
    text-decoration: underline;
    color:var(--color-ocean);
  }
}




// container styles
.container {  
  display: flex;
  flex-direction: column;

  width: 100%;
  padding: 20px;
}

.button-container {
    display: flex;

    margin: 60px auto;

    flex-wrap: wrap;
    justify-content: center;
}




// button styles
.button {
  display: flex;
  overflow: hidden;

  margin: 10px;
  padding: 12px 12px;

  cursor: pointer;
  user-select: none;
  transition: all 150ms linear;
  text-align: center;
  white-space: nowrap;
  text-decoration: none !important;
  text-transform: none;
  text-transform: capitalize;

  color: #fff;
  border: 0 none;
  border-radius: var(--borderRadius);

  font-size: 13px;
  font-weight: 500;
  line-height: 1.3;

  -webkit-appearance: none;
  -moz-appearance:    none;
  appearance:         none;
 
  justify-content: center;
  align-items: center;
  flex: 0 0 160px;
  
  box-shadow: 2px 5px 10px var(--color-smoke);

  &:hover {
    transition: all 150ms linear;

    opacity: .85;
  }
  
  &:active {
    transition: all 150ms linear;
    opacity: .75;
  }
  
  &:focus {
    outline: 1px dotted #959595;
    outline-offset: -4px;
  }
}

.button.-regular {
  color: #202129;
  background-color: #f2f2f2;
  
  &:hover {
    color: #202129;
    background-color: #e1e2e2;
    opacity: 1;
  }
  
  &:active {
    background-color: #d5d6d6;
    opacity: 1;
  }
}

.button.-dark {
  color: var(--color-snow);
  background: var(--color-dark);
  
   &:focus {
    outline: 1px dotted white;
    outline-offset: -4px;
  }
}
  
.button.-green {
  color: var(--color-snow);
  background: var(--color-grass);
}

.button.-blue {
  color: var(--color-snow);
  background: var(--color-ocean);
}

.button.-salmon {
  color: var(--color-snow);
  background: var(--color-salmon);
}

.button.-sun {
  color: #f15c5c;
  background: var(--color-sun);
}

.button.-alge {
  color: #e7ff20;
  background: var(--color-alge);
}

.button.-flower {
  color: #FE8CDF;
  background: var(--color-flower);
}
!

JS 

              
                const	body = document.querySelector('body')
const twitter = document.querySelector('.twitter')

twitter.addEventListener("mouseover", function () {
    body.classList.add('linked')
},false)

twitter.addEventListener("mouseout", function () {
    body.classList.remove('linked')
}, false)
!
999px

Console