<details>
	<summary>Test Dropdown</summary>
	<ul>
		<li>Item 1</li>
		<li>Item 2</li>
		<li>Item 3</li>
		<li>Item 4</li>
		<li>Item 5</li>
		<li>Item 1</li>
		<li>Item 2</li>
		<li>Item 3</li>
		<li>Item 4</li>
		<li>Item 5</li>
	</ul>
</details>

<details class="custom-select">
	<summary class="radios">
		<input type="radio" name="item" id="default" title="Auswählen..." checked>
		<input type="radio" name="item" id="item1" title="Item 1">
		<input type="radio" name="item" id="item2" title="Item 2">
		<input type="radio" name="item" id="item3" title="Item 3">
		<input type="radio" name="item" id="item4" title="Item 4">
		<input type="radio" name="item" id="item5" title="Item 5">
	</summary>
	<ul class="list">
		<li>
			<label for="item1">Item 1</label>
		</li>
		<li>
			<label for="item2">Item 2</label>
		</li>
		<li>
			<label for="item3">Item 3</label>
		</li>
		<li>
			<label for="item4">Item 4</label>
		</li>
		<li>
			<label for="item5">Item 5</label>
		</li>
	</ul>
</details>
html * {
	padding: 0;
	margin: 0;
	box-sizing: border-box;
}

body {
	background-color: #262626;
	display: flex;
	justify-content: center;
	align-items: center;
	height: 100vh;
	font-family: sans-serif;
	padding: 5rem;
}

details {
	position: relative;
	width: 300px;
	margin-right: 1rem;
}

details[open] {
	z-index: 1;
}

summary {
	padding: 1rem;
	cursor: pointer;
	border-radius: 5px;
	background-color: #ddd;
	list-style: none;
}

summary::-webkit-details-marker {
	display: none;
}

details[open] summary:before {
	content: '';
	display: block;
	width: 100vw;
	height: 100vh;
	background: transparent;
	position: fixed;
	top: 0;
	left: 0;
}

summary:after {
	content: '';
	display: inline-block;
	float: right;
	width: .5rem;
	height: .5rem;
	border-bottom: 1px solid currentColor;
	border-left: 1px solid currentColor;
	border-bottom-left-radius: 2px;
	transform: rotate(45deg) translate(50%, 0%);
	transform-origin: center center;
	transition: transform ease-in-out 100ms
}

summary:focus {
	outline: none;
}

details[open] summary:after {
	transform: rotate(-45deg) translate(0%, 0%);
}

ul {
	width: 100%;
	background: #ddd;
	position: absolute;
	top: calc(100% + .5rem);
	left: 0;
	padding: 1rem;
	margin: 0;
	box-sizing: border-box;
	border-radius: 5px;
	max-height: 200px;
	overflow-y: auto;
}

li {
	margin: 0;
	padding: 1rem 0;
	border-bottom: 1px solid #ccc;
}

li:first-child {
	padding-top: 0;
}

li:last-child {
	padding-bottom: 0;
	border-bottom: none;
}

/* FAKE SELECT */

summary.radios {
	counter-reset: radios;
}

input[type=radio] {
	counter-increment: radios;
	appearance: none;
	display: none;
}

input[type=radio]:checked {
	display: inline;
}

input[type=radio]:after {
	content: attr(title);
	display: inline;
	font-size: 1rem;
}

ul.list {
	counter-reset: labels;
}

label {
	width: 100%;
	display: block;
	cursor: pointer;
}

External CSS

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External JavaScript

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