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HTML

About HTML Preprocessors

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

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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).

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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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="example-cntr backdrop">
    <h1>CSS <code>::backdrop</code> pseudo-element</h1>
    <p>The <code>::backdrop</code> pseudo-element is a box that the full screen element has behind it, but at the same time it sits above all other content.</p>
    <p>It can be used together with the <code>:fullscreen</code> pseudo-class or the <code>&lt;dialog></code> HTML5 element to change the background color of the maximized screen, just in case we don’t want to go with the default black.</p>
    <div class="notes">
        <h2>A few notes to consider:</h2>
        <ol>
            <li>It’s mandatory to use double colon with the <code>::backdrop</code> pseudo- element, it doesn’t work with single colon like with other pseudo-elements.</li>
            <li>Edge is the only one that shows any styling of the <code>::backdrop</code> pseudo-element in this demo.</li>
            <li>Firefox 47 (<a href="https://developer.mozilla.org/en-US/Firefox/Releases/47" target="_blank" title="Links opens in a new tab">release date June 29, 2016</a>) will start supporting the <code>::backdrop</code> pseudo-element.</li>
            <li>Also, <a href="http://caniuse.com/#search=dialog" target="_blank"
                    title="Links opens in a new tab">no current version of Firefox supports the <code>&lt;dialog></code></a> HTML5 element.</li>
            <li>Chrome doesn’t style the <code>::backdrop</code> pseudo-element when used with the <code>:fullscreen</code> pseudo-class even though <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/::backdrop#Browser_compatibility" target="_blank"
                    title="Links opens in a new tab">MDN states that Chrome does support it </a>if prefixed with <code>-webkit-</code>. This is for Example 1.</li>
            <li>Chrome properly styles the <code>::backdrop</code> pseudo-element when used with the <code>&lt;dialog></code> HTML5 element. This is for Example 2.</li>
            <li><a href="http://caniuse.com/#search=dialog" target="_blank"
                    title="Links opens in a new tab">Only Chrome supports the <code>&lt;dialog></code></a> HTML5 element at the moment.</li>            
            <li>No mobile device supports the <code>::backdrop</code> pseudo-element.</li>
        </ol>
    </div>
    <div class="example">
        <h2>Example 1: Using <code>:fullscreen</code></h2>
        <h1 id="element"><span class="hide">The text is injected via CSS</span></h1>
        <button onclick="var el = document.getElementById('element'); el.webkitRequestFullscreen();">Trigger Full Screen!</button>
    </div>
    <div class="example">
        <h2>Example 2: Using <code>&lt;dialog></code></h2>
        <dialog>
            <h1>This is a dialog. Good job! :]</h1>
            <p>Press <code>ESC</code> or click on Close button.</p>
            <hr>
            <button id="close">Close</button>
        </dialog>
        <button id="show">Open Dialog!</button>
    </div>
</div>
!

CSS 

              
                //Example 1
#element {
    border-bottom: none;
    background: none;
    //Content in "normal" mode.
    &:before {
        content: 'Normal mode';
    }
    //Content in fullscreen mode
     &:fullscreen:before {
         content: 'Fullscreen mode';         
    }    
    &:fullscreen::backdrop {
        background: $b;
    }
    //According to MDN we need to prefix, but it doesn't work in my tests.
    &:fullscreen::-webkit-backdrop {
        background: $b;
    }
}

//Example 2
dialog {
    width: 350px;
    padding: 40px;
    border: 1px solid rgba(black,.3);
    border-radius: 2px;
    box-shadow: 0 5px 20px black;
    top: 30%;
}
dialog::backdrop {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: rgba(darken($g,25),.8);
}





//Styling stuff not needed for demo
body {
    text-align: left;
    line-height: 1.5;
}
h1 {
    font-size: 22px;
    text-transform: none;
    border-bottom: #636363 1px dotted;
    code {
        display: inline-block;
        margin-bottom: 10px;
        color: $y;
        background: none;
        box-shadow: none;
    }
}
h2 { font-size: 18px; }
code { font-size: 1em; }
.example-cntr {
    max-width: 800px;
    margin: 0 auto 50px;
    padding: 30px;
    box-shadow: 0 1px 2px rgba(black,.3);
    background: rgba(black,.15);
    & > code {
        display: block;
        margin-bottom: 10px;
        font-size: 22px;
        color: $y;
        background: none;
        box-shadow: none;
    }
}
.underline { text-decoration: underline; }

//
.hide { display: none; }
#element {
    border-bottom: #444 1px solid;
    padding-bottom: 20px;
    &:before { 
        display: block;
        padding: 5px 0;
        background: $g;
    }
    &:fullscreen:before {
        display: block;
        padding: 5px 250px;
        background: $r;
    }    
}
button {
    padding: 8px 15px;
    cursor: pointer;
    background: linear-gradient(white, #aaa);
    border-radius: 2px;
    &:hover { background: white; }
    &:active { background: linear-gradient(#aaa, white 20%);}
}
.note {
    color: desaturate($y,30);
    font-size: 12px;
    letter-spacing: .5px;
}
.notes {
    margin-bottom: 20px;
    padding: 20px 20px 0;
    border: #444 1px solid;
    border-radius: 3px;
    h2 { margin: 0; }
}
.example {
    margin: 0 20px;
    padding: 20px;
    background: rgba(black,.2);
    margin-bottom: 20px;
    &:last-child { margin-bottom: 0; }
    h1 { border: none !important; margin-bottom: 0 !important; }
    h2 { margin-top: 0; }
}
dialog {
    text-align: center;
    p { text-align: left; }
}
!

JS 

              
                //Trigger the dialog box
var dialog = document.querySelector('dialog');
document.querySelector('#show').onclick = function() {
  dialog.showModal();
};
document.querySelector('#close').onclick = function() {
  dialog.close();
};

/* More information about the <dialog> element here: http://demo.agektmr.com/dialog/  */
!
999px

Console