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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

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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).

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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 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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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.

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HTML 

              
                <div class="youtube" id="fsrJWUVoXeM" style="width:500px;height:281px;"></div>

<div class="youtube" id="lR4tJr7sMPM" data-params="modestbranding=1&showinfo=0&controls=0&vq=hd720" style="width:640px;height:360px;"></div>

<p class="p">Demo by <a href="https://codepen.io/mancko/" target="_blank">Alexis Ulrich</a>. <a href="http://www.sitepoint.com/faster-youtube-embeds-javascript/" target="_blank">See   article</a>.</p>
!

CSS 

              
                body {
  text-align: center;
  background-color: #1D1F20;
  color: white;
}

a, a:visited {
  color: white;  
}

.youtube {
    background-position: center;
    background-repeat: no-repeat;
    position: relative;
    display: block;
    overflow: hidden;
    transition: all 200ms ease-out;
    cursor: pointer;
    margin: 40px auto 0 auto;
}

iframe {
  margin: 40px auto 0 auto;
}

.youtube .play {
    background: url("data:image/png;base64,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") no-repeat center center;
    background-size: 64px 64px;
    position: absolute;
    height: 100%;
    width: 100%;
    opacity: .8;
    filter: alpha(opacity=80);
    -webkit-transition: all 0.2s ease-out;
    -moz-transition: all 0.2s ease-out;
    -o-transition: all 0.2s ease-out;
    transition: all 0.2s ease-out;
}

.youtube .play:hover {
    opacity: 1;
    filter: alpha(opacity=100);
}

.p {
  margin-top: 60px;
}
!

JS 

              
                "use strict";
function r(f){/in/.test(document.readyState)?setTimeout('r('+f+')',9):f()}
r(function(){
    if(!document.getElementsByClassName) {
        // IE8 support
        var getElementsByClassName = function(node, classname) {
            var a = [];
            var re = new RegExp('(^| )'+classname+'( |$)');
            var els = node.getElementsByTagName("*");
            for(var i=0,j=els.length; i<j; i++)
                if(re.test(els[i].className))a.push(els[i]);
            return a;
        }
        var videos = getElementsByClassName(document.body,"youtube");
    }
    else {
        var videos = document.getElementsByClassName("youtube");
    }

    var nb_videos = videos.length;
    for (var i=0; i<nb_videos; i++) {
        // Based on the YouTube ID, we can easily find the thumbnail image
        videos[i].style.backgroundImage = 'url(http://i.ytimg.com/vi/' + videos[i].id + '/sddefault.jpg)';

        // Overlay the Play icon to make it look like a video player
        var play = document.createElement("div");
        play.setAttribute("class","play");
        videos[i].appendChild(play);

        videos[i].onclick = function() {
            // Create an iFrame with autoplay set to true
            var iframe = document.createElement("iframe");
            var iframe_url = "https://www.youtube.com/embed/" + this.id + "?autoplay=1&autohide=1";
            if (this.getAttribute("data-params")) iframe_url+='&'+this.getAttribute("data-params");
            iframe.setAttribute("src",iframe_url);
            iframe.setAttribute("frameborder",'0');

            // The height and width of the iFrame should be the same as parent
            iframe.style.width  = this.style.width;
            iframe.style.height = this.style.height;

            // Replace the YouTube thumbnail with YouTube Player
            this.parentNode.replaceChild(iframe, this);
        }
    }
});
!
999px

Console