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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.

!
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

Any URLs added here will be added as <link>s in order, and before the CSS in the editor. You can use the CSS from another Pen by using its URL and the proper URL extension.

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About External Resources

You can apply CSS to your Pen from any stylesheet on the web. Just put a URL to it here and we'll apply it, in the order you have them, before the CSS in the Pen itself.

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

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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.

Add External Scripts/Pens

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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.

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

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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About Packages

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.

If you're using React / ReactDOM, make sure to turn on Babel for the JSX processing.

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HTML 

              
                <div class="infobox">
  <div class="header">Lorem Ipsum Dolor Sit Amet</div>
  <div class="text">
    <p><a href="https://jqueryui.com/toggleClass/" target="_blank">View the JQuery UI documentation for Toggle Class.</a> Consectetur adipiscing elit. Ut vel turpis non erat mattis semper. Quisque ac risus nibh. Aenean in arcu quis arcu ultricies auctor. Ut interdum sem sit amet ipsum hendrerit, ut aliquet massa dictum. Ut auctor lacus luctus libero lacinia lacinia. Nam ac arcu orci. Nunc lacinia sem quis posuere aliquam. Sed et ullamcorper est. Mauris augue velit, varius eget malesuada quis, feugiat ac risus. </p>
    <p>Aenean quis arcu sed nisi convallis suscipit. Fusce imperdiet erat eu tincidunt posuere. Duis in libero varius, varius risus lobortis, pellentesque nisi. Sed ut mauris augue. Proin tempus, libero ac efficitur convallis, dolor velit semper nulla, non auctor nisi dolor non odio. Sed rhoncus tristique nibh, nec congue quam bibendum quis. Nam sit amet auctor lectus. Nam pulvinar, risus quis tincidunt placerat, ante dolor luctus orci, non vehicula diam libero sed elit. Morbi quis vehicula libero, rutrum porttitor nisl. Suspendisse et magna et leo tempor volutpat. Pellentesque aliquet eleifend mauris vitae luctus. Nunc aliquam, lacus tristique bibendum sagittis, orci massa vulputate urna, sit amet venenatis eros mi eget magna. Nam dignissim vitae dolor congue sollicitudin.</p>
    <p>Nullam egestas quis est quis malesuada. Curabitur porttitor metus eu libero efficitur tempor. Morbi ex augue, rutrum vel dolor ac, dictum lobortis elit. Nullam sodales, arcu et iaculis vehicula, sapien est pharetra ex, sit amet gravida justo massa in est. Nullam a erat ullamcorper, auctor nunc vitae, luctus ligula. Pellentesque eu eleifend mi. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Mauris sed orci sapien. Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Maecenas faucibus posuere sem et commodo. Nulla interdum, nulla ac accumsan vehicula, augue libero elementum magna, non venenatis erat felis sed ex. Quisque mattis tincidunt lectus, sit amet aliquet libero. In efficitur ante urna, non porta sem auctor sit amet. Nulla facilisi. Integer rhoncus tincidunt quam eu venenatis.</p>
    <p>Donec hendrerit bibendum magna, ut aliquet erat lobortis nec. Maecenas in volutpat ex, eget facilisis nisi. Vivamus finibus tellus libero, ac aliquam velit dapibus quis. Aliquam in odio sagittis, tempor enim et, pharetra leo. Morbi quis convallis mauris. Praesent laoreet imperdiet commodo. Mauris neque tellus, ornare vel maximus vel, porttitor vitae ante. In sodales maximus nisl nec varius. Etiam ligula leo, vestibulum quis faucibus eget, dapibus at est. Ut imperdiet dolor eu magna dictum, eu faucibus ex porta. Aliquam placerat dapibus dolor, ac dapibus dui cursus sit amet.</p>
    <p>Phasellus felis ligula, hendrerit vel elit nec, maximus pharetra lectus. Integer convallis eros ipsum, id ornare ex pharetra et. Vestibulum dapibus risus ligula, in pharetra turpis gravida ac.</p>
  </div>
  <div class="controls">
    <button type="button" id="js-read-more-btn">Toggle Panel</button>
  </div>
</div>
!

CSS 

              
                html {
  font-size: 16px;
}

body {
  background-color: #e2e2e2;
  color: #454545;
}

button {
  font-size: 1rem;
  background-color: #006699;
  border: none 0 transparent;
  border-radius: .3rem;
  padding: .5rem .75rem;
  color: #dedede;
  font-weight: bold;
}

.infobox {
  font-size: 1rem;
  max-width: 45rem;
  margin: 0 auto;
  border: 1px solid #006699;
  background-color: azure;
  margin-top: 1rem;
  padding: 1.5rem 2rem;
}

.infobox .header {
  font-size: 1.5rem;
  margin-bottom: 1rem;
  font-weight: bold;
}

.infobox .text {
  height: 100px;
  line-height: 1.3;
  overflow: hidden;
}

.infobox .text.is-open {
  height: auto;
}

.infobox .controls {
  margin-top: 1rem;
  text-align: right;
}
!

JS 

              
                $("#js-read-more-btn").click(function() {
  $(".infobox .text").toggleClass("is-open", 250);
});
!
999px

Console