<form>
	<label for="os">OS</label>
	<select id="os" name="os">
		<option value="none" selected>Select…</option>
		<option value="linux">Linux</option>
		<option value="macos">macOS</option>
		<option value="windows">Windows</option>
	</select>
	<label for="fruit">Fruit</label>
	<select id="fruit" name="fruit">
		<option value="none" selected>Select…</option>
		<option value="apple">Apple</option>
		<option value="banana">Banana</option>
		<option value="cherry">Cherry</option>
		<option value="durian">Durian</option>
	</select>
</form>
* {
	border: 0;
	box-sizing: border-box;
	margin: 0;
	padding: 0;
}
:root {
	--foldDur: 0.7s;
	font-size: calc(24px + (30 - 24)*(100vw - 320px)/(1280 - 320));
}
body, button {
	color: #171717;
	font: 1em Gaegu, cursive;
	line-height: 1.5;
}
body {
	background-image:
		linear-gradient(#8ccfd100 1.35em,#8ccfd1 1.4em 1.45em,#8ccfd100 1.5em),
		linear-gradient(90deg,#e5e5e5 1.35em,#8ccfd1 1.4em 1.45em,#e5e5e5 1.5em);
	background-position: 50% 0;
	background-size: 1.5em 1.5em;
}
form, .select__button, .select__options {
	width: 100%;
}
form {
	margin: auto;
	padding: 3em 1.5em 6em 1.5em;
	max-width: 13.25em;
}
label {
	display: block;
	font-weight: bold;
}
.select, .select__button {
	position: relative;
}
.select {
	margin-bottom: 1.5em;
	perspective: 20em;
}
.select select {
	display: none;
}
.select__button, .select__option {
	background: #f1f1f1;
	cursor: pointer;
	display: flex;
	justify-content: space-between;
	align-items: center;
	padding: 0 0.375em;
}
.select__button {
	animation-duration: var(--foldDur);
	animation-timing-function: ease-in-out;
	animation-direction: reverse;
	border-radius: 0;
	box-shadow:
		-0.05em 0 0 #fff inset,
		0.05em 0 0 #ccc inset,
		0 -0.05em 0 #d9d9d9 inset;
	transform-origin: 50% 100%;
	-webkit-appearance: none;
	appearance: none;
	z-index: 1;
}
.select__button:focus, .select__option:focus {
	outline: transparent;
}
.select__button:focus {
	color: #2762f3;
}
.select__button:after, .select__option:first-child:after {
	border-left: 0.25em solid transparent;
	border-right: 0.25em solid transparent;
	content: "";
	display: block;
	width: 0;
	height: 0;
}
.select__button:after {
	border-top: 0.25em solid;
}
.select__button--pristine {
	animation: none;
}
.select__options {
	position: absolute;
	top: 0;
	left: 0;
	visibility: hidden;
	transition: visibility 0s var(--foldDur) steps(1);
}
.select__option {
	box-shadow:
		-0.05em -0.1em 0.2em 0 #0001 inset,
		0.05em 0 0 #ccc inset,
		-0.05em 0.1em 0.2em 0 #fff inset,
		-0.05em 0 0 #fff inset;
	color: inherit;
	opacity: 0;
	text-decoration: none;
	transition: opacity 0s var(--foldDur) steps(1);
}
.select__option:first-child {
	box-shadow:
		-0.05em -0.1em 0.2em 0 #0001 inset,
		0.05em 0 0 #ccc inset,
		-0.05em 0 0 #fff inset;
}
.select__option:first-child:after {
	border-bottom: 0.25em solid;
}
.select__option:hover, .select__option:focus {
	background: #b8ccf9;
	box-shadow:
		-0.05em -0.1em 0.2em 0 #0001 inset,
		0.05em 0 0 #a9bbe5 inset,
		-0.05em 0 0 #c1d4ff inset;
}
.select__option[aria-selected="true"]:not(:first-child):after {
	content: "\2713";
}

/* Animation */
.select__button[aria-expanded="true"] {
	animation-direction: normal;
	animation-fill-mode: forwards;
	box-shadow:
		-0.05em -0.1em 0.2em 0 #fff inset,
		-0.05em 0 0 #fff inset;
	cursor: default;
	pointer-events: none;
}
.select__button--fold4 {
	animation-name: fold4;
}
.select__button--fold5 {
	animation-name: fold5;
}
.select__button[aria-expanded="true"] + .select__options,
.select__button[aria-expanded="true"] + .select__options .select__option {
	transition-delay: 0s;
}
.select__button--fold4[aria-expanded="true"] + .select__options,
.select__button--fold5[aria-expanded="true"] + .select__options {
	visibility: visible;
}
.select__button--fold4[aria-expanded="true"] + .select__options .select__option,
.select__button--fold5[aria-expanded="true"] + .select__options .select__option {
	opacity: 1;
}

.select__button--fold4 + .select__options .select__option:nth-child(4),
.select__button--fold4[aria-expanded="true"] + .select__options .select__option:nth-child(2) {
	transition-delay: calc(var(--foldDur) * 0.25);
}
.select__button--fold4 + .select__options .select__option:nth-child(3),
.select__button--fold4[aria-expanded="true"] + .select__options .select__option:nth-child(3) {
	transition-delay: calc(var(--foldDur) * 0.5);
}
.select__button--fold4 + .select__options .select__option:nth-child(2),
.select__button--fold4[aria-expanded="true"] + .select__options .select__option:nth-child(4) {
	transition-delay: calc(var(--foldDur) * 0.75);
}

.select__button--fold5 + .select__options .select__option:nth-child(5),
.select__button--fold5[aria-expanded="true"] + .select__options .select__option:nth-child(2) {
	transition-delay: calc(var(--foldDur) * 0.2);
}
.select__button--fold5 + .select__options .select__option:nth-child(4),
.select__button--fold5[aria-expanded="true"] + .select__options .select__option:nth-child(3) {
	transition-delay: calc(var(--foldDur) * 0.4);
}
.select__button--fold5 + .select__options .select__option:nth-child(3),
.select__button--fold5[aria-expanded="true"] + .select__options .select__option:nth-child(4) {
	transition-delay: calc(var(--foldDur) * 0.6);
}
.select__button--fold5 + .select__options .select__option:nth-child(2),
.select__button--fold5[aria-expanded="true"] + .select__options .select__option:nth-child(5) {
	transition-delay: calc(var(--foldDur) * 0.8);
}

@keyframes appear {
	from { opacity: 0 }
	to { opacity: 1 }
}
@keyframes fold4 {
	from { color: #171717; transform: translateY(0) rotateX(0deg) }
	12.5% { color: #171717; transform: translateY(0) rotateX(-90deg) }
	12.51% { color: transparent; transform: translateY(0) rotateX(-90deg) }
	25% { color: transparent; transform: translateY(0) rotateX(-180deg) }
	25.01% { color: transparent; transform: translateY(100%) rotateX(0deg) }
	50% { color: transparent; transform: translateY(100%) rotateX(-180deg) }
	50.01% { color: transparent; transform: translateY(200%) rotateX(0deg) }
	75% { color: transparent; transform: translateY(200%) rotateX(-180deg) }
	75.01% { color: transparent; transform: translateY(300%) rotateX(0deg) }
	to { color: transparent; transform: translateY(300%) rotateX(-100deg) }
}
@keyframes fold5 {
	from { color: #171717; transform: translateY(0) rotateX(0deg) }
	10% { color: #171717; transform: translateY(0) rotateX(-90deg) }
	10.01% { color: transparent; transform: translateY(0) rotateX(-90deg) }
	20% { color: transparent; transform: translateY(0) rotateX(-180deg) }
	20.01% { color: transparent; transform: translateY(100%) rotateX(0deg) }
	40% { color: transparent; transform: translateY(100%) rotateX(-180deg) }
	40.01% { color: transparent; transform: translateY(200%) rotateX(0deg) }
	60% { color: transparent; transform: translateY(200%) rotateX(-180deg) }
	60.01% { color: transparent; transform: translateY(300%) rotateX(0deg) }
	80% { color: transparent; transform: translateY(300%) rotateX(-180deg) }
	80.01% { color: transparent; transform: translateY(400%) rotateX(0deg) }
	to { color: transparent; transform: translateY(400%) rotateX(-100deg) }
}
document.addEventListener("DOMContentLoaded",() => {
	const os = new SelectDropdown({ id: "os" }),
		fruit = new SelectDropdown({ id: "fruit" });
});

class SelectDropdown {
	constructor(args) {
		this.isOpening = false;
		this.select = document.querySelector(`select[id=${args.id}]`);
		this.selectBtn = document.createElement("button");
		this.options = document.createElement("div");
		this.buildDropdown();
	}
	buildDropdown() {
		if (this.select !== null) {
			// create div to contain <select>
			let wrapper = document.createElement("div");
			wrapper.setAttribute("class","select");
			this.select.parentElement.insertBefore(wrapper,this.select);
			wrapper.appendChild(this.select);

			// create button
			let id = this.select.id,
				selectBtnAttrs = {
					"class": "select__button select__button--pristine",
					"type": "button",
					"id": `${id}-options`,
					"aria-haspopup": "listbox",
					"aria-expanded": "false"
				};

			for (let a in selectBtnAttrs)
				this.selectBtn.setAttribute(a,selectBtnAttrs[a]);

			let selectBtnText = document.createTextNode(this.select.options[0].innerHTML);
			this.selectBtn.appendChild(selectBtnText);
			wrapper.appendChild(this.selectBtn);

			// create options div
			let optionsAttrs = {
				"class": "select__options",
				"aria-labelledby": selectBtnAttrs.id
			};
			for (let a in optionsAttrs)
				this.options.setAttribute(a,optionsAttrs[a]);

			// then each option
			for (let o of this.select.options) {
				let option = document.createElement("a"),
					optionText = document.createTextNode(o.innerHTML),
					optionAttrs = {
						"href": "#",
						"class": "select__option",
						"data-value": o.value,
						"role": "option",
						"aria-selected": "false"
					};

				for (let a in optionAttrs)
					option.setAttribute(a,optionAttrs[a]);

				option.appendChild(optionText);
				this.options.appendChild(option);
			}
			wrapper.appendChild(this.options);

			// sync with pre-selected option
			let preselected = this.options.querySelector(`[data-value=${this.select.value}]`);
			preselected.setAttribute("aria-selected",true);
			this.selectBtn.innerHTML = preselected.innerHTML;

			// restack so options can appear over other dropdowns
			let selects = document.querySelectorAll(".select"),
				selectCount = 0;
			while (selectCount < selects.length) {
				selects[selectCount].style.zIndex = selects.length - selectCount;
				++selectCount;
			}

			// assign event listeners
			document.querySelector(`label[for=${id}]`).addEventListener("click",() => {
				if (!this.isExpanded())
					this.selectBtn.focus();
				else
					this.closeSelect();
			});
			this.selectBtn.addEventListener("click",() => { this.openSelect(); });
			this.options.addEventListener("click",e => { this.closeSelect(e); });

			document.addEventListener("click",() => {
				let el = document.activeElement;
				if (el.parentElement.getAttribute("aria-labelledby") !== selectBtnAttrs.id)
					this.closeSelect();
			});
			window.addEventListener("keydown",e => {
				switch (e.keyCode) {
					case 27: // Esc
						this.closeSelect();
						break;
					case 32: // Spacebar
						this.closeSelect(e);
						break;
					case 38: // Up
						this.goToOption("previous");
						break;
					case 40: // Down
						this.goToOption("next");
						break;
					default:
						break;
				}
			});
		}
	}
	isExpanded() {
		return this.selectBtn.getAttribute("aria-expanded") === "true";
	}
	openSelect(e) {
		if (!this.isExpanded()) {
			// prevent immediate closing to not ruin animation
			let foldDur = window.getComputedStyle(this.options);
			foldDur = foldDur.getPropertyValue("transition-delay").split("");
			if (foldDur.indexOf("m") > -1) {
				foldDur.splice(foldDur.length - 2,2);
				foldDur = parseInt(foldDur.join(""));

			} else if (foldDur.indexOf("s") > -1) {
				foldDur.pop();
				foldDur = parseFloat(foldDur.join("")) * 1e3;
			}
			this.isOpening = true;
			setTimeout(() => {this.isOpening = false;},foldDur);

			// manage states
			this.selectBtn.setAttribute("aria-expanded",true);

			let btnClasses = this.selectBtn.classList,
				pristineClass = "select__button--pristine",
				animClass = `select__button--fold${this.select.options.length}`;

			if (btnClasses.contains(pristineClass)) {
				btnClasses.remove(pristineClass);
			} else {
				btnClasses.remove(animClass);
				void this.selectBtn.offsetWidth;
			}
			btnClasses.add(animClass);

			// set focus to selected option
			let selected = this.options.querySelector("[aria-selected=true]");
			if (selected !== null)
				selected.focus();
			else
				this.options.childNodes[0].focus();
		}
	}
	closeSelect(e) {
		if (this.isExpanded() && !this.isOpening) {
			if (e && e.target !== this.options.childNodes[0]) {
				// update values of both original and custom dropdowns
				this.select.value = e.target.getAttribute("data-value");
				this.selectBtn.innerHTML = e.target.innerHTML;

				// indicate selected item
				for (let n of this.options.childNodes)
					n.setAttribute("aria-selected",false);

				e.target.setAttribute("aria-selected",true);
				e.preventDefault();
			}
			this.selectBtn.setAttribute("aria-expanded",false);
			this.selectBtn.focus();

			// fire animation
			let btnClasses = this.selectBtn.classList,
				animClass = `select__button--fold${this.select.options.length}`;

			btnClasses.remove(animClass);
			void this.selectBtn.offsetWidth;
			btnClasses.add(animClass);
		}
	}
	goToOption(goTo) {
		if (this.isExpanded()) {
			let optionLinks = this.options.querySelectorAll("a"),
				activeEl = document.activeElement,
				linkFound = false;

			// check for focused option
			for (let l of optionLinks) {
				if (activeEl === l) {
					linkFound = true;
					break;
				}
			}
			// allow movement with arrows until top or bottom option is reached
			if (linkFound) {
				if (goTo === "previous" && activeEl !== optionLinks[0])
					activeEl.previousSibling.focus();

				else if (goTo === "next" && activeEl !== optionLinks[optionLinks.length - 1])
					activeEl.nextSibling.focus();
			}
		}
	}
}
Run Pen

External CSS

  1. https://fonts.googleapis.com/css?family=Gaegu:400,700&amp;display=swap

External JavaScript

This Pen doesn't use any external JavaScript resources.