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HTML

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Adding Classes

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About the <head>

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Insecure Resource

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CSS

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CSS Base

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                

		
<div id="container">

<div class="highlight"></div>
	
<div class="outer">
	<div class="inner">
		<div class="icon">1</div>
		<div class="text">
			hello
		</div>
	</div>
</div>

<div class="outer">
	<div class="inner">
		<div class="icon">2</div>
		<div class="text">
			it&rsquo;s me
		</div>
	</div>
</div>

<div class="outer">
	<div class="inner">
		<div class="icon">3</div>
		<div class="text">
			I&rsquo;ve been wonderin&rsquo; somethin&rsquo; somethin&rsquo; me
		</div>
	</div>
</div>
	
	<div class="outer">
	<div class="inner">
		<div class="icon">4</div>
		<div class="text">
			hello from the outside
		</div>
	</div>
</div>
	
	<div class="outer">
		<div class="inner">
		<div class="icon">5</div>
		<div class="text">
			at least I can say that I tried
		</div>
	</div>
</div>
	
	<div class="outer">
		<div class="inner">
		<div class="icon">6</div>
		<div class="text">
			to get these lyrics right
		</div>
	</div>
</div>
	
</div>
!

CSS 

              
                $umit: 40px;
$marg: 12px;
$t: .5s;
$bpx: 0px;
$bg: #7de8df;
$accent: #a8db18;

html,body{
	margin: 0;
	padding: 0;
	height: 100%;
}
body{
	display: flex;
	align-items: center;
	justify-content: center;
	background: $bg;
}

.inner{
	display: flex;
	align-items: center;
}

.text{
	white-space: nowrap;
	display: flex;
	align-items: center;
	padding-right: 2*$marg;
}

.icon{
	width: $umit - 2*$bpx;
	height: $umit - 2*$bpx;
	display: flex;
	align-items: center;
	justify-content: center;
}

.outer{
	display: flex;
	width: $umit;
	height: $umit;
	overflow: hidden;
	transition: $t;
	margin: $marg;
	border-radius: 1000px;
	border: $bpx solid rgba(0,0,0,.0);
	box-shadow: inset 0px 3px 3px 1px rgba(0,0,0,.08),inset 0px -1px 2px 1px rgba(255,255,255,.45);
	cursor: pointer;
}

*{
	flex-shrink: 0;
}

#container{
	display: inline-flex;
	position: relative;
	// background: #fafafa;
	border-radius: 1000px;
	box-shadow: inset 0px -2px 5px 0 rgba(0,0,0,.1),inset 0px 2px 5px 0 rgba(255,255,255,.5);
	background: rgba(255,255,255,.2);
}

#container::after{
	content: '';
	position: absolute;
	width: 100%;
	height: 10px;
	border-radius: 100%;
	bottom: 0px;
	box-shadow: 0 40px 20px 3px rgba(0,0,0,.13);
	transform: scaleX(1.05);
}

.highlight{
	height: $umit;
	width: $umit;
	position: absolute;
	z-index: 1;
	background: $accent;
	margin-top: $marg;
	left: $marg;
	transition: $t;
	border-radius: 1000px;
	border: 1px solid rgba(0,0,0,.1);
	box-shadow: inset 0 -5px 8px 0 rgba(0,0,0,.2),inset 0 5px 8px 0 rgba(255,255,255,.5), 0 1px 3px 0 rgba(0,0,0,.1);
}

.selected{
	width: initial;
}

.inner{
	position: relative;
}
.active .inner{
	z-index: 2;
}

*{
	font-family: "Heebo", sans-serif;
	color: white;
	font-weight: 700;
	box-sizing: border-box;
}

.icon{
	color: rgba(0,0,0,.4);
	font-weight: 900;
}

.text{
	text-shadow: 0 1px rgba(0,0,0,.4)
}

.outer:not(.active) .text{
	visibility: hidden;
}
!

JS 

              
                const outers = Array.from(document.getElementsByClassName('outer'));
let active = null;
let queue = Promise.resolve();

const h = document.getElementsByClassName('highlight')[0];
const init = outers[0].style.width;

const u = outers[1].getBoundingClientRect().x - outers[0].getBoundingClientRect().x;

outers.forEach((o,i) => {
	o.dataset.idx = i;
	o.addEventListener('click', ()=>activate(o));
})

function activate(thing){
	if (active === thing) return;
	queue = queue.then(() => {
		return new Promise(r => {
			moveTo(thing);
			minimize(thing).then(()=>expand(thing)).then(r);
			active = thing;
			thing.classList.add('active');
		})
	})
}


function minimize(thing){
	if (!active) return Promise.resolve();
	active.classList.remove('active');
	return new Promise(r => {
		active.addEventListener('transitionend',r,{once:true})
		active.style.width = init;
		h.style.width = init;
	});
}

function moveTo(target){
	h.style.transform = `translateX(${u*target.dataset.idx}px)`
}

function expand(target){
	return new Promise(r => {
		target.addEventListener('transitionend',r,{once: true});
		const width = target.getElementsByClassName('inner')[0].getBoundingClientRect().width + 'px';
		target.style.width = width;
		h.style.width = width;
	})
}

setTimeout(()=>activate(outers[0]),500)
!
999px

Console