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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.

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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).

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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 

              
                <!-- horizontal -->

<h4>Horizontal Flip</h4>
<div class="flip">
    <div class="front" style="background-image: url(https://images.pexels.com/photos/540518/pexels-photo-540518.jpeg?w=1260&h=750&dpr=2&auto=compress&cs=tinysrgb)">
       <h1 class="text-shadow">MOUNTAIN</hi>
    </div>
    <div class="back">
       <h2>Angular</h2>
       <p>Good tools make application development quicker and easier to maintain than if you did everything by hand..</p>
    </div>
</div>
<div class="flip">
    <div class="front" style="background-image: url(https://images.pexels.com/photos/414171/pexels-photo-414171.jpeg?w=1260&h=750&dpr=2&auto=compress&cs=tinysrgb)">
       <h1 class="text-shadow">LAKE</hi>
    </div>
    <div class="back">
       <h2>Angular</h2>
       <p>Good tools make application development quicker and easier to maintain than if you did everything by hand..</p>
    </div>
</div>
<div class="flip">
    <div class="front" style="background-image: url(https://images.pexels.com/photos/36717/amazing-animal-beautiful-beautifull.jpg?w=1260&h=750&dpr=2&auto=compress&cs=tinysrgb)">
       <h1 class="text-shadow">OCEAN</hi>
    </div>
    <div class="back">
       <h2>Angular</h2>
       <p>Good tools make application development quicker and easier to maintain than if you did everything by hand..</p>
    </div>
</div>

<br>
<br>

<!-- vertical -->
  
<h4>Vertical Flip</h4>
<div class="flip flip-vertical">
    <div class="front" style="background-image: url(https://images.pexels.com/photos/38136/pexels-photo-38136.jpeg?w=1260&h=750&dpr=2&auto=compress&cs=tinysrgb)">
       <h1 class="text-shadow">FOREST</hi>
    </div>
    <div class="back">
       <h2>Angular</h2>
       <p>Good tools make application development quicker and easier to maintain than if you did everything by hand..</p>
    </div>
</div>
<div class="flip flip-vertical">
    <div class="front" style="background-image: url(https://images.pexels.com/photos/34950/pexels-photo.jpg?w=1260&h=750&dpr=2&auto=compress&cs=tinysrgb)">
       <h1 class="text-shadow">TRACK</hi>
    </div>
    <div class="back">
       <h2>Angular</h2>
       <p>Good tools make application development quicker and easier to maintain than if you did everything by hand..</p>
    </div>
</div>
<div class="flip flip-vertical">
    <div class="front" style="background-image: url(https://images.pexels.com/photos/34546/sunset-lake-landscape-summer.jpg?w=1260&h=750&dpr=2&auto=compress&cs=tinysrgb)">
       <h1 class="text-shadow">SWAMP</hi>
    </div>
    <div class="back">
       <h2>Angular</h2>
       <p>Good tools make application development quicker and easier to maintain than if you did everything by hand..</p>
    </div>
</div>
!

CSS 

              
                @import url('https://fonts.googleapis.com/css?family=Roboto+Mono');

* {
  box-sizing: border-box;
  font-weight: normal;
}

body {
  color: #555;
  background: #222;
  text-align: center;
  font-family: 'Roboto Mono';
  padding: 1em;
}

h1 {
  font-size: 2.2em;
}

// base
.flip {
    position: relative;
    >.front,
    >.back {
        display: block;
        transition-timing-function: cubic-bezier(.175, .885, .32, 1.275);
        transition-duration: .5s;
        transition-property: transform, opacity;
    }
    >.front {
        transform: rotateY(0deg);
    }
    >.back {
        position: absolute;
        opacity: 0;
        top: 0px;
        left: 0px;
        width: 100%;
        height: 100%;
        transform: rotateY(-180deg);
    }
    &:hover {
        >.front {
            transform: rotateY(180deg);
        }
        >.back {
            opacity: 1;
            transform: rotateY(0deg);
        }
    }
    &.flip-vertical {
        >.back {
            transform: rotateX(-180deg);
        }
        &:hover {
            >.front {
                transform: rotateX(180deg);
            }
            >.back {
                transform: rotateX(0deg);
            }
        }
    }
}

// custom
.flip {
    position: relative;
    display: inline-block;
    margin-right: 2px;
    margin-bottom: 1em;
    width: 400px;
    >.front,
    >.back {
      display: block;
      color: white;
      width: inherit;
      background-size: cover!important;
      background-position: center!important;
      height: 220px;
      padding: 1em 2em;
      background: #313131;
      border-radius: 10px;
      p {
        font-size: 0.9125rem;
        line-height: 160%;
        color: #999;
      }
    }
}

.text-shadow {
text-shadow: 1px 1px rgba(0, 0, 0, 0.04), 2px 2px rgba(0, 0, 0, 0.04), 3px 3px rgba(0, 0, 0, 0.04), 4px 4px rgba(0, 0, 0, 0.04), 0.125rem 0.125rem rgba(0, 0, 0, 0.04), 6px 6px rgba(0, 0, 0, 0.04), 7px 7px rgba(0, 0, 0, 0.04), 8px 8px rgba(0, 0, 0, 0.04), 9px 9px rgba(0, 0, 0, 0.04), 0.3125rem 0.3125rem rgba(0, 0, 0, 0.04), 11px 11px rgba(0, 0, 0, 0.04), 12px 12px rgba(0, 0, 0, 0.04), 13px 13px rgba(0, 0, 0, 0.04), 14px 14px rgba(0, 0, 0, 0.04), 0.625rem 0.625rem rgba(0, 0, 0, 0.04), 16px 16px rgba(0, 0, 0, 0.04), 17px 17px rgba(0, 0, 0, 0.04), 18px 18px rgba(0, 0, 0, 0.04), 19px 19px rgba(0, 0, 0, 0.04), 1.25rem 1.25rem rgba(0, 0, 0, 0.04);
}
!

JS 

              
                // no js !!
!
999px

Console