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HTML 

              
                <main>
	<h1>CSS-Only Animated Accordion</h1>
	<section class="accordion" id="overview">
		<h1 class="title"><a href="#overview">Overview</a></h1>
		<div class="content">
			<div class="wrapper">
				<p>
					This CodePen demonstrates how we can animate the opening and closing
					states of an accordion with fancy reveal animation using only CSS. This
					concept is suitable for creating <strong>FAQ sections</strong>,
					<strong>Table of Contents</strong>, and more.
				</p>
			</div>
		</div>
	</section>

	<section class="accordion" id="how-does-it-work">
		<h1 class="title"><a href="#how-does-it-work">How does it work?</a></h1>
		<div class="content">
			<div class="wrapper">
				<p>
					We cannot transition height or any CSS property from zero to auto. If we
					want to smoothly expand a collapsed accordion, we could set a
					<code>max-height</code> and transition the <code>height</code> property
					to a large value when we expand the section. However,
					<a href="https://css-tricks.com/using-css-transitions-auto-dimensions/#aa-there-are-two-crucial-downsides-to-this" target="_blank">it may not properly work</a>
					if the content is larger than the <code>max-height</code>.
				</p>
				<p>
					So, here we create a CSS grid with a grid item. Then we transition the
					<code>grid-template-rows</code> property from <code>0fr</code> to
					<code>1fr</code> and the grid item transitions to its content height.
				</p>
				<p>
					I used CSS <code>clip-path</code> and <code>mix-blend-mode</code> to
					animate the background and text color of the accordion. Each accordion
					has two pseudo-elements. We know that <code>opacity</code>,
					<code>clip-path</code> and, <code>visibility</code> are animatable CSS
					properties. So when the accordion is selected using the
					<code>:target</code> pseudo-class, we transition the clip-path circle
					radius to 200% of the
					<code>::before</code>
					pseudo-element. When the accordion is not selected, we do the same with
					the
					<code>::after</code> but this time with a delay. This creates an
					illusion as it appears to clip from inside.
				</p>
				<p>
					Since the accordion's title is an anchor element, users can also
					navigate through the items with their keyboard.
				</p>
			</div>
		</div>
	</section>

	<section class="accordion" id="inspiration">
		<h1 class="title"><a href="#inspiration">Inspiration</a></h1>
		<div class="content">
			<div class="wrapper">
				<p>
					In a recent video, Kevin showed us a way to animate height from zero to
					auto value. Seeing that, I couldn't resist experimenting with the idea.
					Then I came up with this CSS-only accordion component and added the
					reveal animation too!
				</p>
				<p>
					I would highly recommend watching
					<a href="https://youtu.be/B_n4YONte5A" target="_blank">his video</a>,
					where he beautifully explains the technique and points out how he
					discovered it. Also, read
					<a href="https://keithjgrant.com/posts/2023/04/transitioning-to-height-auto/" target="_blank">Keith J. Grant's post</a>
					and
					<a href="https://nemzes.net/posts/animating-height-auto/" target="_blank">Nelson Menezes's post</a>
					on the same topic.
				</p>
			</div>
		</div>
	</section>
</main>
!

CSS 

              
                :root {
	--size-header: 2.25rem;
	--size-accordion-title: 1.25rem;
	--size-accordion-content: 1rem;
	--animation-speed: 100;
	--slide-ease: cubic-bezier(0.86, 0, 0.07, 1);
	--slide-duration: calc(400ms * 100 / var(--animation-speed));
	--slide-delay: calc(450ms * 100 / var(--animation-speed));
	--circle-duration: calc(900ms * 100 / var(--animation-speed));
}

*,
*::before,
*::after {
	position: relative;
	left: 0;
	top: 0;
	box-sizing: border-box;
}

a,
p,
h1 {
	margin: 0;
}

html {
	height: 100%;
}

body {
	background-color: hsl(4, 91%, 60%);
	font-family: Nunito, Arial, Helvetica, sans-serif;
	font-weight: 600;
	margin: 0;
	display: grid;
	place-items: center;
	padding: 2rem 2rem;
	min-height: 100%;
}

html,
body {
	scroll-behavior: smooth;
	scroll-padding-top: 1rem;
}

main > h1 {
	font-size: var(--size-header);
	margin-bottom: 1.25rem;
	color: #fff;
}

::selection {
	background-color: rgba(0, 0, 0, 0.4);
}

.accordion {
	--circle-x: 1.8rem;
	--circle-y: 0;
	--circle-r: 200%;
	--circle-bg: #fff;
	color: #000;

	background-color: var(--circle-bg);
	max-width: 56ch;
	margin-bottom: 1rem;
	border-radius: min(8px, 0.5rem);

	display: grid;
	grid-template-rows: 0fr 0fr;
	transition-timing-function: var(--slide-ease);
	transition-duration: 200ms, 200ms, var(--slide-duration);
	transition-property: opacity, box-shadow, grid-template-rows;
	transition-delay: 0ms, 0ms, var(--slide-delay);
	box-shadow: 0 0 0 2px rgba(0, 0, 0, 0.1);
	opacity: 0.9;
}

.accordion:not(:target):hover {
	box-shadow: 0 0 0 2px rgba(0, 0, 0, 0.5);
}

.accordion:not(:target):active {
	opacity: 1;
	box-shadow: 0 4px 7px 0 rgba(0, 0, 0, 0.3);
}

.accordion,
.content {
	overflow: hidden;
}

.accordion:target {
	--d: 90deg;
	grid-template-rows: 0fr 1fr;
	transition: grid-template-rows var(--slide-ease) var(--slide-duration)
		var(--slide-delay);
}

.wrapper {
	padding-block: 0 1.05rem;
	padding-inline: 1.25rem;
}

.content {
	font-size: var(--size-accordion-content);
	line-height: 140%;
}

.content p {
	margin-bottom: 1rem;
}

.content a {
	color: currentColor;
	font-weight: 800;
	text-decoration: underline;
}

main :last-child,
.content :last-child {
	margin-bottom: 0;
}

.title a {
	padding: 1rem 1.25rem;
	font-size: var(--size-accordion-title);
	font-weight: 800;
	color: currentColor;
	text-decoration: none;
	display: flex;
	flex-direction: row;
	place-items: center;
}

.title a::before {
	--chevron-icon: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 320 512'%3E%3C!--! Font Awesome Pro 6.4.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license (Commercial License) Copyright 2023 Fonticons, Inc. --%3E%3Cpath fill='white' d='M310.6 233.4c12.5 12.5 12.5 32.8 0 45.3l-192 192c-12.5 12.5-32.8 12.5-45.3 0s-12.5-32.8 0-45.3L242.7 256 73.4 86.6c-12.5-12.5-12.5-32.8 0-45.3s32.8-12.5 45.3 0l192 192z'/%3E%3C/svg%3E");
	content: "";
	left: 0;
	top: 0;
	width: 0.65rem;
	aspect-ratio: 320 / 512;
	display: inline-block;
	margin-right: 0.75rem;
	transform: rotate(var(--d, 0deg));
	transition: transform var(--slide-ease) var(--slide-duration)
		var(--slide-delay);
	mask-image: var(--chevron-icon);
	mask-size: 100% 100%;
	-webkit-mask-image: var(--chevron-icon);
	-webkit-mask-size: 100% 100%;
	background-color: currentColor;
}

.accordion::before,
.accordion::after {
	content: "";
	position: absolute;
	width: 100%;
	height: 100%;
	background-color: var(--circle-bg);
	mix-blend-mode: difference;
	transform-style: preserve-3d;
	transition-timing-function: ease;
	transition-property: opacity, clip-path, visibility;
	pointer-events: none;
	clip-path: circle(var(--r) at var(--circle-x) var(--circle-y));
	border-radius: inherit;
	z-index: 4;
}

.accordion::before {
	--r: 0%;
	transition-delay: var(--circle-duration), var(--circle-duration), 0ms;
	transition-duration: 0ms, var(--circle-duration), 0ms;
	opacity: 0;
}

.accordion:target::before {
	--r: var(--circle-r);
	transition-delay: 0ms, 0ms, 0ms;
	transition-duration: 0ms, var(--circle-duration), 0ms;
	opacity: 1;
}

.accordion::after {
	--r: var(--circle-r);
	transition-delay: 0ms, 0ms, var(--circle-duration);
	transition-duration: 0ms, var(--circle-duration), 0ms;
	visibility: hidden;
	opacity: 1;
}

.accordion:target:after {
	--r: 0%;
	transition-delay: 0ms, 0ms, 0ms;
	transition-duration: 0ms, 0ms, 0ms;
	visibility: visible;
	opacity: 0;
}

.title a:focus-visible {
	background-color: hsl(0, 100%, 90%);
	outline: none;
}

.accordion:target .title a:focus-visible {
	background-color: hsl(183, 100%, 93%);
}
!

JS
!
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