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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.

     

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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-.

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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.

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You're using npm packages, so we've auto-selected Babel for you here, which we require to process imports and make it all work. If you need to use a different JavaScript preprocessor, remove the packages in the npm tab.

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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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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>
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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);
}
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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) });
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