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HTML 

              
                <div class="scroller">
	<header>
		<div class="container">
			<h1>Intersection Observer Visualizer</h1>
			<p class="subheading">A simple visualizer tool to assist with using the <a href="https://developer.mozilla.org/en-US/docs/Web/API/Intersection_Observer_API">Intersection Observer API</a>.</p>
			<p>Adjust the <code>rootMargin</code> and <code>threshold</code> values. The two shaded boxes show where the elements’ <code>intersectionRatio</code> matches the <code>threshold</code> value*.</p>
			<aside class="header__aside">
				<p><small><strong>*Note: </strong><code>threshold</code> can optionally take an array instead of a number. For simplicity, this visualizer only includes a single value option.</small></p>
			</aside>
		</div>
	</header>
	<main>
		<div class="grid">
			<div class="grid__input-col">
				<aside class="input-group">
					<h3>Options</h3>
					<div class="input-group__row">
						<label class="input-group__label" for="margin">rootMargin: <span class="input-group__value" data-margin></span></label>
						<input class="input-group__range" type="range" id="margin" name="margin" min="-30" max="200" step="10" value="120" data-input="margin">
					</div>
					<div class="input-group__row">
						<label class="input-group__label" for="margin">threshold: <span class="input-group__value" data-threshold></span></label>
						<input class="input-group__range" type="range" id="threshold" name="threshold" min="0" max="1" step="0.1" value="10" data-input="threshold">
					</div>
					<div class="info" data-info aria-live="polite"></div>
				</aside>
			</div>

			<div class="grid__observer">
				<div class="outer-wrapper">
					<div class="wrapper" data-wrapper>
							<section>
								<div class="box" data-box="1">1</div>
							</section>
							<section>
								<div class="box" data-box="2">2</div>
							</section>
							<section>
								<div class="box" data-box="3">3</div>
							</section>
							<section>
								<div class="box" data-box="4">4</div>
							</section>
							<section>
								<div class="box" data-box="5">5</div>
							</section>
						</div>
				</div>
			</div>
		</div>
	</main>
</div>
<footer>&copy; Michelle Barker</footer>
!

CSS 

              
                @import url('https://fonts.googleapis.com/css2?family=Poppins:[email protected];600&display=swap');

* {
	box-sizing: border-box;
}

:root {
	--b: 10px;
	--box: max(10rem, 20vmin);
	--outerPad: 12rem;
	--primaryColor: deeppink;
	--sliderColor: var(--primaryColor);
	--sliderBorder: white;
	--sliderThumbColor: white;
	
	--ltBg: #f2f4f5;
	--mdBg: #e1e6e8;
	--dkBg: #303947;
	--text: #1d2026;
}

body {
	font-family: 'Poppins', sans-serif;
	font-weight: 300;
	margin: 0;
	padding: 1rem 0;
	width: 100%;
	background-color: var(--ltBg);
	color: var(--text);
	
	@media (min-width: 50em) {
		font-size: 1.2rem;
		padding: 0;
	}
}

// .scroller {
// 	@media (min-width: 75em) {
// 		@supports (scroll-snap-type: y mandatory) {
// 			height: 100vh;
// 			overflow-y: scroll;
// 			scroll-snap-type: y mandatory;

// 			> * {
// 				scroll-snap-align: start;
// 			}
// 		}
// 	}
// }

h1 {
	@media (min-width: 60em) {
		font-size: 3rem;
	}
}

h1,
h3 {
	margin: 0 0 1rem;
}

p {
	max-width: 70ch;
	line-height: 1.6;
}

strong {
	font-weight: 600;
}

a {
	color: var(--primaryColor);
	transition: color 200ms;
	
	&:hover,
	&:focus {
		color: black;
	}
}

header {
	padding: 1rem 0 2rem;
	
	@media (min-width: 80em) {
		margin-bottom: 1rem;
		padding: 2rem 0;
	}
}

.header__aside {
	background-color: var(--mdBg);
	padding: 0.125rem 1rem;
	border-radius: 0.2rem;
	max-width: 70ch;
}

code {
	background-color: #cbd3d6;
	padding: 0.125rem;
}

.subheading {
	font-weight: 600;
	
	@media (min-width: 50em) {
		font-size: 1.4rem;
	}
}

.container {
	max-width: 120rem;
	margin: 0 auto;
	padding: 0 1rem;
	
	@media (min-width: 50em) {
		padding: 0 2rem;
	}
}

.grid {
	@media (min-width: 75em) {
		display: grid;
		grid-template-columns: 32rem 2fr;
		gap: 1rem;
		background-color: var(--mdBg);
	}
	
	@media (min-width: 100em) {
		grid-template-columns: 1fr 2fr;
		min-height: 100vh;
	}
}

.grid__observer {
	padding: var(--outerPad);
	background-color: var(--mdBg);
}

section {
	height: 85vh;
	max-height: 35rem;
	display: flex;
	align-items: center;
	justify-content: center;
}

.box {
	position: relative;
	width: var(--box);
	height: var(--box);
	background-color: rgba(112, 230, 214, 1);
	color: white;
	padding: 1rem;
	display: flex;
	align-items: center;
	justify-content: center;
	transition: background-color 200ms;
	counter-increment: section;
	
	&.is-inview {
		background-color: var(--primaryColor);
	}
	
	&::after {
		content: counter(section);
		position: absolute;
		top: 0.5rem;
		left: 50%;
		transform: translate3d(-50%, 0, 0);
		padding: 0.25rem;
		width: 1.2rem;
		height: 1.2rem;
		font-size: 0.85rem;
		border-radius: 50%;
		background-color: rgba(0, 0, 0, 0.2);
		display: flex;
		align-items: center;
		justify-content: center;
	}
}

.outer-wrapper {
	position: relative;
	width: 100%;
	max-width: 90vw;
	height: calc(80vh - 12rem);
	min-height: 25rem;
	max-height: 40rem;
	margin: 0 auto;
	outline: var(--b) solid paleturquoise;
	transition: outline 200ms;
	
	&::after,
	&::before {
		--y: calc(100% - var(--t));
		display: block;
		content: '';
		position: absolute;
		width: var(--box);
		height: var(--box);
		left: 50%;
		transform: translate3d(-50%, 0, 0);
		background: rgba(0, 0, 0, 0.25);
		pointer-events: none;
		transition: transform 200ms, top 200ms, bottom 200ms;
		will-change: top, left;
		z-index: 1;
	}
	
	&::before {
		--offsetY: calc(var(--y) * -1);
		top: calc(var(--b) * -1);
		transform: translate3d(-50%, var(--offsetY, 0), 0);
	}
	
	&::after {
		bottom: calc(var(--b) * -1);
		top: auto;
		transform: translate3d(-50%, var(--y, 0), 0) rotate(180deg);
	}
	
	@media (min-width: 75em) {
		max-width: 65rem;
	 	margin: 0 auto 0 0;
	}
}

.wrapper {
	width: 100%;
	height: 100%;
	overflow: scroll;
	border: 2px solid;
	outline-offset: var(--b);
	outline: 2px dashed var(--text);
	transition: outline-offset 200ms;
	background-color: white;
	counter-reset: section;
}

.grid__input-col {
	@media (min-width: 75rem) {
		background-color: var(--dkBg);
	}
}

.input-group {
	padding: 1rem;
	background-color: var(--dkBg);
	color: white;
	z-index: 1;
	
	@media (min-width: 50em) {
		font-size: 1rem;
	}
	
	@media (min-width: 75rem) {
		margin: var(--outerPad) 0 0;
		align-self: start;
		max-width: 31rem;
		margin-left: auto;
		padding-right: 3rem;
	}
}

.input-group__row {
	display: flex;
	flex-wrap: wrap;
	align-items: center;
	max-width: 31rem;
}

.input-group__label {
	display: flex;
	flex: 1 0 10rem;
	margin-right: 1rem;
}

.input-group__range {
	flex: 0 0 15rem;
}

.input-group__value {
	font-weight: 600;
	margin-left: auto;
}

.info {
	margin: 2rem 0 2rem;
	background-color: #1d2026;
	padding: 0.5rem 1rem;
	
	p {
		margin: 0;
	}
}

.info__elements {
	font-weight: 600;
	color: var(--primaryColor);
	padding-left: 0.5rem;
}

footer {
	background-color: var(--text);
	color: var(--mdBg);
	font-size: 0.9rem;
	padding: 0.5rem 1rem;
	
	@media (min-width: 50rem) {
		padding: 0.5rem 2rem;
	}
}


/* Range input styles (http://danielstern.ca) */
input[type=range] {
  -webkit-appearance: none;
  width: 100%;
  margin: 5px 0;
	background: var(--inputBg);
}

input[type=range]:focus {
  outline: none;
}

input[type=range]::-webkit-slider-runnable-track {
	width: 100%;
  height: 6px;
  cursor: pointer;
  background: var(--sliderColor);
}

input[type=range]::-webkit-slider-thumb {
  box-shadow: 0 0 0.4rem rgba(0, 0, 0, 0.5);
  border: 1px solid var(--sliderBorder);
  height: 20px;
  width: 20px;
  border-radius: 50%;
  background: var(--sliderThumbColor);
  cursor: pointer;
  -webkit-appearance: none;
  margin-top: -7px;
}

input[type=range]:focus::-webkit-slider-runnable-track {
  background: #8080ff;
}


input[type=range]::-moz-range-track {
  width: 100%;
  height: 6px;
  cursor: pointer;
  background: var(--sliderColor);
}

input[type=range]::-moz-range-thumb {
  box-shadow: 0 0 0.4rem rgba(0, 0, 0, 0.5);
  border: 1px solid var(--sliderBorder);
  height: 20px;
  width: 20px;
  border-radius: 50%;
  background: var(--sliderThumbColor);
  cursor: pointer;
}

input[type=range]::-ms-track {
  width: 100%;
  height: 6px;
  cursor: pointer;
  background: transparent;
  border-color: transparent;
  color: transparent;
}

input[type=range]::-ms-fill-lower {
  background: var(--sliderColor);
  border: 0px solid var(--sliderBorder);;
}

input[type=range]::-ms-fill-upper {
  background: var(--sliderColor);
  border: 0px solid var(--sliderBorder);
}

input[type=range]::-ms-thumb {
  box-shadow: 0 0 0.4rem rgba(0, 0, 0, 0.5);
  border: 1px solid var(--sliderBorder);
  height: 20px;
  width: 20px;
  border-radius: 21px;
  background: var(--sliderThumbColor);
  cursor: pointer;
}

input[type=range]:focus::-ms-fill-lower {
  background: #8080ff;
}

input[type=range]:focus::-ms-fill-upper {
  background: #8080ff;
}
!

JS 

              
                const marginInput = document.querySelector('[data-input="margin"]')
const thresholdInput = document.querySelector('[data-input="threshold"]')
const wrapper = document.querySelector('[data-wrapper]')
const boxes = [...document.querySelectorAll('[data-box]')]
const marginText = document.querySelector('[data-margin]')
const thresholdText = document.querySelector('[data-threshold]')
const info = document.querySelector('[data-info]')

const getOptions = () => {
	return {
		root: wrapper,
		rootMargin: `${marginInput.value}px`,
		threshold: parseFloat(thresholdInput.value)
	}
}

let elementsInView = []

const getInfo = () => {
	if (elementsInView.length) {
		return `<p>Element indexes in view: <span class="info__elements">${elementsInView.join(', ')}</span></p>`
	} else {
		return '<p>No elements in view</p>'
	}
}

const addElementToInfoBox = (target) => {
	if (!elementsInView.includes(target.dataset.box)) {
		elementsInView.push(target.dataset.box)
	}
	
	return info.innerHTML = getInfo()
}

const removeElementFromInfoBox = (target) => {
	elementsInView = elementsInView.filter(el => el !== target.dataset.box)
	
	return info.innerHTML = getInfo()
}

const setInViewStyles = (target) => {
	target.textContent = 'In view'
	target.classList.add('is-inview')
	addElementToInfoBox(target)
	// console.log('in view')
}

const setOutOfViewStyles = (target) => {
	target.textContent = ''
	target.classList.remove('is-inview')
	removeElementFromInfoBox(target)
	// console.log('out of view')
}

const onIntersect = (entries) => {
	entries.forEach(entry => {
		const { target, isIntersecting, intersectionRatio } = entry
		console.log(entry)
		
		if (intersectionRatio >= thresholdInput.value && isIntersecting) {
			return setInViewStyles(target)
		}
		
		return setOutOfViewStyles(target)
	})
}

const reinitObserver = (options) => {
	if (observer) {
		observer.disconnect()
	}
	
	setTimeout(() => {
		observer = new IntersectionObserver(onIntersect, options)
	
		boxes.forEach(el => {
			observer.observe(el)
		})
	}, 100)
}

const setMargin = (marginValue) => {
	const newValue = `${marginInput.value}px`
	document.body.style.setProperty('--b', marginValue)
	marginText.textContent = marginValue
}

const setThreshold = (thresholdValue) => {
	const tValue = `${thresholdValue * 100}%`
	thresholdText.textContent = thresholdValue
	document.body.style.setProperty('--t', tValue)
}

const onChangeThreshold = () => {
	const opts = getOptions()
	setThreshold(opts.threshold)
	reinitObserver(opts)
}

const onChangeMargin = () => {
	const opts = getOptions()
	setMargin(opts.rootMargin)
	reinitObserver(opts)
}

let observer = new IntersectionObserver(onIntersect, getOptions())

boxes.forEach(el => {
	observer.observe(el)
})

setMargin(getOptions().rootMargin)
setThreshold(getOptions().threshold)
marginInput.addEventListener('input', onChangeMargin)
thresholdInput.addEventListener('input', onChangeThreshold)
!
999px

Console