Pen Settings

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.

Learn more · Versions

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.

Learn more · Versions

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. If you link to another Pen, it will include the CSS from that Pen. If the preprocessor matches, it will attempt to combine them before processing.

Powered by Algolia
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.

If the stylesheet 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 CSS 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.

Learn more

+ add another resource

JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient. For instance, CoffeeScript can help prevent easy-to-make mistakes and offer a cleaner syntax and Babel can bring ECMAScript 6 features to browsers that only support ECMAScript 5.

Learn more · Versions

Babel is required to process package imports. If you need a different preprocessor remove all packages first.

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.

Powered by Algolia
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.

Learn more

+ add another resource

Behavior

Save Automatically?

If active, Pens will autosave every 30 seconds after being saved once.

Auto-Updating Preview

If enabled, the preview panel updates automatically as you code. If disabled, use the "Run" button to update.

Format on Save

If enabled, your code will be formatted when you actively save your Pen. Note: your code becomes un-folded during formatting.

Editor Settings

Code Indentation

Want to change your Syntax Highlighting theme, Fonts and more?

Visit your global Editor Settings.

HTML 

              
                <h1>Scroll down 👇</h1>

<div class="box"></div>
<div class="box"></div>
<div class="box"></div>
<div class="box"></div>
<div class="box"></div>
<div class="box"></div>
<div class="box"></div>
<div class="box"></div>
<div class="box"></div>
<div class="box"></div>
!

CSS 

              
                :root {
	--base: #202050;
	--border: #ccc;
}

body {
	display: flex;
	align-items: center;
	flex-direction: column;
	font-family: -apple-system,BlinkMacSystemFont,Segoe UI,Helvetica,Arial,sans-serif,Apple Color Emoji,Segoe UI Emoji;
	color: var(--base);
	margin-top: 20vh;
}

.box {
	width: 30vh;
	height: 30vh;
	margin: 20vh 0;
	border: 0.2rem solid var(--border);
	border-radius: 50%;
	transition: background 0.8s ease,
		border 0.4s ease;
}

.box.isVisible {
	border-color: var(--base);
	background-color: var(--base);
}
!

JS 

              
                // Options docs: https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API#Intersection_observer_options
const options = {
  root: null, // use the document's viewport as the container
  rootMargin: '0px', // % or px - offsets added to each side of the intersection 
  threshold: 0.5 // percentage of the target element which is visible
}

// Callback docs: https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API#Targeting_an_element_to_be_observed
let callback = (entries) => { 
	entries.forEach(entry => {
		
		// If entry (box) is visible - according with the params set in `options`
		// then adds `isVisible` class to box
		// otherwise removes `isVisible` class
		
		if(entry.isIntersecting) {
			entry.target.classList.add('isVisible');
		} else {
			entry.target.classList.remove('isVisible');		
		}

	});
}

// Create the intersection observer instance by calling its constructor and passing it a
// callback function to be run whenever a threshold is crossed in one direction or the other:
let observer = new IntersectionObserver(callback, options);

// Get all the `.box` from DOM and attach the observer to these
document.querySelectorAll('.box')
	.forEach(box => { observer.observe(box) });
!
999px

Console