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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 URL's 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 it's URL and the proper URL extention.

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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

Any URL's added here will be added as <script>s in order, and run before the JavaScript in the editor. You can use the URL of any other Pen and it will include the JavaScript from that Pen.

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About External Resources

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 Skypack, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ES6 import 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>
  <p>Click on this first visible paragraph.</p>
  <p>It's harder when the content you want to animate the height of is supposed to be initially closed. 
    Nam at tortor porta, imperdiet mi at, iaculis lorem. Donec mollis faucibus lacinia. Ut mollis tortor vitae lorem malesuada pretium. Suspendisse porta malesuada dui interdum mattis. In malesuada, enim id pulvinar luctus, eros lectus posuere nunc, et eleifend risus velit ac ipsum. Morbi urna est, ornare quis ligula egestas, cursus euismod eros. Cras eu malesuada lacus, at lacinia velit. Fusce a lorem ultrices nunc gravida consectetur. Phasellus vel mollis quam. Vestibulum bibendum, velit quis auctor congue, lectus arcu ultricies velit, vel ullamcorper orci libero ac neque. Suspendisse in orci vitae diam vulputate finibus eget eget felis. Sed elementum sit amet urna a posuere. Cras volutpat sagittis est, eget tempus leo interdum ac. Vestibulum accumsan dui at ornare rutrum. Phasellus orci lectus.</p>
</div>
<hr/>
!

CSS 

              
                p{}
p:first-child{
  text-decoration: underline
}
p:last-child{
  --fullHeight: auto;
  height: var(--fullHeight);
  opacity: 0;
}
p.ready{
  height: 0;
  overflow: hidden;
  transition:all 1s ease-in
}
p.ready.open{
  height: var(--fullHeight);
  opacity: 1;
}
!

JS 

              
                /* this is jQuery code  but simple in plain JS */

$(document).ready(function() {
  
  var p1 = $("p:first-child");
  var p2 = $("p:last-child");
  
  $(p2)[0].style.setProperty("--fullHeight",  $(p2).height() + "px");
  
  $(p2).addClass("ready");
  
  $(p1).on("click",function(e){
    
    $(p2).toggleClass("open");
    
  })
  
});
!
999px

Console