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HTML 

              
                <main>

	<h1>Pure CSS Accordion</h1>
	<div class="accordion">
		<input type="radio" name="accordion" id="accordion-1" checked="">
		<label for="accordion-1">Accordion One</label>
		<div class="accordion-content">
			<p><b>Accordion Content - 1</b></p>
			<p>Made using pseudo selector radio inputs:checked + labels for tabs, and + selector to target adjacent siblings accordion content.</p>
			<p>*Remove checked="" on the first accordion if you want to start with a fully closed accordion.</p>
			<p>Super simple, and easy to modify for your own needs.</p>
		</div>

		<input type="radio" name="accordion" id="accordion-2">
		<label for="accordion-2">Accordion Two</label>
		<div class="accordion-content">
			<p><b>Accordion Content - 2</b></p>
			<p>Lorem ipsum dolor sit amet consectetur adipisicing elit.<br/>Fugit deserunt aliquam quia dicta qui natus error aut molestiae nostrum nihil possimus iure, veniam architecto aspernatur id nulla, incidunt ducimus suscipit.</p>
			<p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Fugit deserunt aliquam quia dicta qui natus error aut molestiae nostrum </p>
		</div>

		<input type="radio" name="accordion" id="accordion-3">
		<label for="accordion-3">Accordion Three</label>
		<div class="accordion-content">
			<p><b>Accordion Content - 3</b></p>
			<p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Fugit deserunt aliquam quia dicta qui natus error aut molestiae nostrum nihil possimus iure, veniam architecto.</p>
			<p>Lorem ipsum dolor sit amet consectetur adipisicing elit. Fugit deserunt aliquam quia dicta qui natus error aut molestiae nostrum nihil possimus iure, veniam architecto.Lorem ipsum dolor sit amet consectetur adipisicing elit.</p>
		</div>

	</div>
</main>
    <footer>
        <p>Developed by Stephen Fray 🏄 | <a href="https://codepen.io/stepfray" target="_blank" rel="noreferrer" title="View My Code Pen Profile">CodePen</a> | <a href="https://www.behance.net/stephen_fray" arget="_blank" rel="noreferrer" title="View my Behance Profile">Behance</a></p>
    </footer>
!

CSS 

              
                ////////////////////
// Colour Vars
////////////////////
$light: #eee;
$light-shade-2: #ccc;
$light-shade-3: #bababa;
$dark: #444444;

////////////////////
// Accordion
////////////////////
.accordion {
	display: flex;
	flex-direction: column;
	flex-wrap: wrap;
}

////////////////////
// Accordion -- Hide Radios Inputs
////////////////////
.accordion input[type="radio"] {
	display: none;
}

////////////////////
// Accordion -- Section Titles (labels)
////////////////////
.accordion label {
	padding: 0.3em 1em;
	cursor: pointer;
	display: inline-flex;
	&:not(:last-of-type) {
		border-bottom: 1px solid $light-shade-3;
	}
	&:first-of-type {
		border-top-left-radius: 10px;
		border-top-right-radius: 10px;
	}
	&:last-of-type {
		border-bottom-left-radius: 10px;
		border-bottom-right-radius: 10px;
	}
}
.accordion label {
	user-select: none;
	background-color: $light-shade-2;
}
.accordion label:hover {
	background: $light-shade-2;
}
.accordion input:checked + label {
	background: $light-shade-2;
	border-bottom: 1px solid $light;
	&:last-of-type {
		border-bottom-left-radius: 0;
		border-bottom-right-radius: 0;
	}
}

////////////////////
// Accordion -- Content
////////////////////
.accordion-content {
	background-color: $light;
	overflow: hidden;
	&:last-of-type {
		border-bottom-left-radius: 10px;
		border-bottom-right-radius: 10px;
	}
	p {
		margin-bottom: 0;
		&:first-of-type {
			margin-top: 0;
		}
	}
}

////////////////////
// Accordion -- Show/Hide Content
////////////////////
.accordion-content {
	max-height: 0;
	visibility: hidden;
	transition: all 0.8s;
	padding: 0 1.2em;
	* {
		opacity: 0;
		transition: opacity 0.8s;
	}
}
.accordion input[type="radio"]:checked + label + .accordion-content {
	max-height: 100vh;
	visibility: visible;
	opacity: 1;
	padding: 1.2em;
	* {
		opacity: 1;
	}
}

////////////////////
// Page Set Up - Ignore
////////////////////
* {
	box-sizing: border-box;
}
body {
	color: $dark;
	display: grid;
	place-content: center;
	padding: 0;
	margin: 0;
	min-height: 100vh;
	background-image: #99c5ff;
	background-image: radial-gradient(
			at 32% 10%,
			hsla(29, 95%, 71%, 1) 0,
			transparent 52%
		),
		radial-gradient(at 46% 54%, hsla(179, 98%, 69%, 1) 0, transparent 47%),
		radial-gradient(at 33% 9%, hsla(199, 73%, 71%, 1) 0, transparent 53%),
		radial-gradient(at 63% 58%, hsla(200, 77%, 77%, 1) 0, transparent 52%),
		radial-gradient(at 60% 91%, hsla(45, 70%, 78%, 1) 0, transparent 56%),
		radial-gradient(at 14% 87%, hsla(297, 77%, 69%, 1) 0, transparent 51%),
		radial-gradient(at 81% 56%, hsla(309, 85%, 62%, 1) 0, transparent 40%),
		radial-gradient(at 70% 57%, hsla(261, 98%, 77%, 1) 0, transparent 40%),
		radial-gradient(at 37% 48%, hsla(260, 85%, 60%, 1) 0, transparent 54%),
		radial-gradient(at 54% 28%, hsla(230, 77%, 69%, 1) 0, transparent 56%),
		radial-gradient(at 22% 83%, hsla(54, 91%, 76%, 1) 0, transparent 58%),
		radial-gradient(at 58% 34%, hsla(170, 96%, 62%, 1) 0, transparent 58%),
		radial-gradient(at 43% 93%, hsla(59, 92%, 79%, 1) 0, transparent 42%),
		radial-gradient(at 3% 23%, hsla(125, 72%, 78%, 1) 0, transparent 41%),
		radial-gradient(at 77% 92%, hsla(24, 82%, 64%, 1) 0, transparent 58%),
		radial-gradient(at 62% 97%, hsla(53, 87%, 72%, 1) 0, transparent 54%),
		radial-gradient(at 84% 48%, hsla(133, 81%, 78%, 1) 0, transparent 42%);
	/// Background by Mesher
	/// Spread the love: https://www.csshero.org/mesher/
	font-family: apple-system, sans-serif;
	font-size: 18px;
	line-height: 1.5;
	padding-bottom: 50vh;
}
main {
	max-width: 700px;
	margin: auto;
	padding: clamp(15px, 2vw, 60px);
}
h1 {
	font-size: clamp(1.5rem, 7vw, 2.5rem);
}
p {
	font-size: clamp(0.9rem, 3vw, 1.1rem);
}
label {
	font-size: clamp(1rem, 3vw, 1.4rem);
}
footer {
    position: fixed;
    bottom: 0;
    left: 0;
    right: 0;
    text-align: center;
    background-color: rgba(255, 255, 255, 0.5);
    padding: 10px;
	z-index:9999999;
    p {
        font-size: 0.8rem;
        margin: 0;
    }
}
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