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HTML

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HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

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Adding Classes

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CSS

About CSS Preprocessors

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

About Vendor Prefixing

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JavaScript

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JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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HTML 

              
                <form>
	<input type="checkbox" name="lever" class="lever pristine" id="lever" value="lever value" role="switch" aria-label="lever" aria-checked="false">
	<label for="lever"><span>On</span></label>
	<label for="lever"><span>Off</span></label>
</form>
!

CSS 

              
                *, *:before, *:after {
	box-sizing: border-box;
	margin: 0;
	padding: 0;
}
:root {
	--dur: 0.3s;
	--dur2: 0.15s;
	font-size: 16px;
}
body {
	background-color: #dbdfdb;
}
form {
	display: flex;
	justify-content: space-between;
	align-items: center;
	margin: 3em auto 0 auto;
	width: 23em;
}
form label {
	background-color: #f00;
	background-image:
		radial-gradient(0.3em 0.25em at 50% 25%, rgb(255,255,255) 25%, rgba(255,255,255,0)),
		radial-gradient(0.25em 0.25em at 30% 75%, rgba(255,255,255,0.5), rgba(255,255,255,0)),
		radial-gradient(0.3em 0.3em at 60% 80%, rgba(255,255,255,0.5), rgba(255,255,255,0)),
		radial-gradient(100% 100%, rgba(255,255,255,0) 30%, rgba(255,255,255,0.3) 40%, rgba(0,0,0,0.5) 50%);
	border-radius: 50%;
	box-shadow: 0 0 0.75em #f00, 0 0.5em 0.75em rgba(0,0,0,0.3);
	display: inline-block;
	transition: all var(--dur2) var(--dur2) linear;
	width: 1.5em;
	height: 1.5em;
}
form label:first-of-type {
	background-color: #050;
	box-shadow: 0 0.5em 0.75em rgba(0,0,0,0.3);
	order: -1;
}
label span {
	display: inline-block;
	overflow: hidden;
	width: 0;
}
/* Lever */
.lever {
	background-image:
		radial-gradient(1em 1.5em at 50% 50%, rgb(204,204,204) 25%, rgb(170,170,170) 49%, rgba(170,170,170,0) 50%),
		radial-gradient(0.65em 1em at 50% 53%, rgb(68,68,68), rgba(68,68,68,0)),
		radial-gradient(0.75em 0.75em at 55% 45%, rgb(255,255,255), rgba(255,255,255,0)),
		radial-gradient(0.5em 0.4em at 43% 56%, rgba(255,255,255,0.5), rgba(255,255,255,0)),
		radial-gradient(0.5em 0.4em at 55% 57%, rgba(255,255,255,0.6), rgba(255,255,255,0)),
		radial-gradient(0.5em 0.4em at 50% 60%, rgba(255,255,255,0.6), rgba(255,255,255,0)),
		radial-gradient(100% 100% at 50% 50%, rgb(238,238,238), rgb(170,170,170) 10%, rgb(0,0,0) 18%, rgb(0,0,0) 21%, rgb(153,153,153) 21.5%, rgb(153,153,153) 24%, rgba(153,153,153,0) 24.5%, rgba(221,221,221,0) 30%, rgb(221,221,221) 30.5%, rgb(221,221,221) 33%, rgb(68,68,68) 33.5%, rgb(68,68,68) 35%, rgba(68,68,68,0) 35.5%, rgba(221,221,221,0) 41%, rgb(221,221,221) 41.5%, rgb(238,238,238) 45%, rgb(68,68,85) 45.5%, rgb(68,68,85) 49%, rgba(68,68,85,0) 50%),
		radial-gradient(2em 2em at 35% 20%, rgb(255,255,255), rgba(255,255,255,0) 2em),
		radial-gradient(2em 2em at 65% 80%, rgb(255,255,255), rgba(255,255,255,0) 2em),
		radial-gradient(2em 2em at 80% 35%, rgb(255,255,255), rgba(255,255,255,0) 2em),
		radial-gradient(2em 2em at 20% 65%, rgb(255,255,255), rgba(255,255,255,0) 2em),
		radial-gradient(100% 100% at center, rgb(170,170,170) 49%, rgba(170,170,170,0) 50%);
	background-position: 0.5em 0, 0.5em 0, 0 0, 0 0, 0 0, 0 0, 0 0, 0 0, 0 0, 0 0, 0 0, 0 0;
	border-radius: 50%;
	box-shadow: 0 2em 2em rgba(0,0,0,0.3);
	cursor: pointer;
	font-size: 1em;
	display: block;
	outline: 0;
	position: relative;
	width: 12em;
	height: 12em;
	-webkit-appearance: none;
	-moz-appearance: none;
	appearance: none;
	transition: background-position var(--dur) linear;
}
.lever:before, .lever:after {
	content: "";
	display: block;
	position: absolute;
	top: 50%;
	left: 50%;
}
.lever:before {
	animation: leverAOff var(--dur) linear forwards;
	background:
		radial-gradient(35% 25% at 50% 25%, rgb(136,136,153) 20%, rgba(136,136,153,0)),
		radial-gradient(1em 0.75em at 32% 75%, rgb(102,102,102), rgba(102,102,102,0)),
		radial-gradient(1em 0.75em at 65% 78%, rgb(102,102,102), rgba(102,102,102,0)),
		radial-gradient(100% 100% at center, rgb(68,68,68) 49.5%, rgba(68,68,68,0) 50%);
	box-shadow:
		-2em 4em 2em rgba(0,0,0,0.3),
		-2em 5em 4em rgba(0,0,0,0.2),
		-2em 7em 4em rgba(0,0,0,0.2),
		-2em 9em 4em rgba(0,0,0,0.2),
		-2em 11em 4em rgba(0,0,0,0.2);
	border-radius: 50%;
	width: 4em;
	height: 4em;
	transform: translate(100%,-50%);
	z-index: 1;
}
.lever:checked {
	background-position: -0.5em 0, -0.5em 0, 0 0, 0 0, 0 0, 0 0, 0 0, 0 0, 0 0, 0 0, 0 0, 0 0;
}
.lever:checked:before {
	animation: leverAOn var(--dur) linear forwards;
}
.lever:after {
	animation: leverBOff var(--dur) linear forwards;
	background-color: #333;
	border-radius: 15em;
	box-shadow: 0 0 1em #667 inset;
	width: 7.5em;
	height: 4em;
	transform: translate(0.5em,-50%);
}
.lever:checked:after {
	animation: leverBOn var(--dur) linear forwards;
}
.lever:checked + label {
	background-color: #0d0;
	box-shadow: 0 0 0.75em #0f0, 0 0.5em 0.75em rgba(0,0,0,0.3);
}
.lever:checked + label + label {
	background-color: #500;
	box-shadow: 0 0.5em 0.75em rgba(0,0,0,0.3);
}
/* To stop animations from running on load */
.pristine:before, .pristine:after {
	animation: none;
}

@keyframes leverAOn {
	from {
		box-shadow:
			-2em 4em 2em rgba(0,0,0,0.3),
			-2em 5em 4em rgba(0,0,0,0.2),
			-2em 7em 4em rgba(0,0,0,0.2),
			-2em 9em 4em rgba(0,0,0,0.2),
			-2em 11em 4em rgba(0,0,0,0.2);
		transform: translate(100%,-50%);
	}
	50% {
		box-shadow:
			0 6em 2em rgba(0,0,0,0.3),
			0 5em 4em rgba(0,0,0,0.2),
			0 7em 4em rgba(0,0,0,0.2),
			0 9em 4em rgba(0,0,0,0.2),
			0 11em 4em rgba(0,0,0,0.2);
	}
	to {
		box-shadow:
			2em 4em 2em rgba(0,0,0,0.3),
			2em 5em 4em rgba(0,0,0,0.2),
			2em 7em 4em rgba(0,0,0,0.2),
			2em 9em 4em rgba(0,0,0,0.2),
			2em 11em 4em rgba(0,0,0,0.2);
		transform: translate(-200%,-50%);
	}
}
@keyframes leverAOff {
	from {
		box-shadow:
			2em 4em 2em rgba(0,0,0,0.3),
			2em 5em 4em rgba(0,0,0,0.2),
			2em 7em 4em rgba(0,0,0,0.2),
			2em 9em 4em rgba(0,0,0,0.2),
			2em 11em 4em rgba(0,0,0,0.2);
		transform: translate(-200%,-50%);
	}
	50% {
		box-shadow:
			0 6em 2em rgba(0,0,0,0.3),
			0 5em 4em rgba(0,0,0,0.2),
			0 7em 4em rgba(0,0,0,0.2),
			0 9em 4em rgba(0,0,0,0.2),
			0 11em 4em rgba(0,0,0,0.2);
	}
	to {
		box-shadow:
			-2em 4em 2em rgba(0,0,0,0.3),
			-2em 5em 4em rgba(0,0,0,0.2),
			-2em 7em 4em rgba(0,0,0,0.2),
			-2em 9em 4em rgba(0,0,0,0.2),
			-2em 11em 4em rgba(0,0,0,0.2);
		transform: translate(100%,-50%);
	}
}
@keyframes leverBOn {
	from { width: 7.5em; transform: translate(0.5em,-50%); }
	50% { width: 4em; transform: translate(-50%,-50%); }
	to { width: 7.5em; transform: translate(-8em,-50%); }
}
@keyframes leverBOff {
	from { width: 7.5em; transform: translate(-8em,-50%); }
	50% { width: 4em; transform: translate(-50%,-50%); }
	to { width: 7.5em; transform: translate(0.5em,-50%); }
}
!

JS 

              
                document.querySelector(".lever").addEventListener("change", function(){
	this.classList.remove("pristine");
	
	let ac = "aria-checked";
	this.setAttribute(ac,this.getAttribute(ac) == "true" ? "false" : "true");
});
!
999px

Console