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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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About the <head>

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Insecure Resource

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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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Packages

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HTML 

              
                <input type="checkbox" id="darkmode-toggle" />
<label for="darkmode-toggle">
	<svg version="1.1" class="sun" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" viewBox="0 0 496 496" style="enable-background:new 0 0 496 496;" xml:space="preserve">

		<rect x="152.994" y="58.921" transform="matrix(0.3827 0.9239 -0.9239 0.3827 168.6176 -118.5145)" width="40.001" height="16" />
		<rect x="46.9" y="164.979" transform="matrix(0.9239 0.3827 -0.3827 0.9239 71.29 -12.4346)" width="40.001" height="16" />
		<rect x="46.947" y="315.048" transform="matrix(0.9239 -0.3827 0.3827 0.9239 -118.531 50.2116)" width="40.001" height="16" />

		<rect x="164.966" y="409.112" transform="matrix(-0.9238 -0.3828 0.3828 -0.9238 168.4872 891.7491)" width="16" height="39.999" />

		<rect x="303.031" y="421.036" transform="matrix(-0.3827 -0.9239 0.9239 -0.3827 50.2758 891.6655)" width="40.001" height="16" />

		<rect x="409.088" y="315.018" transform="matrix(-0.9239 -0.3827 0.3827 -0.9239 701.898 785.6559)" width="40.001" height="16" />

		<rect x="409.054" y="165.011" transform="matrix(-0.9239 0.3827 -0.3827 -0.9239 891.6585 168.6574)" width="40.001" height="16" />
		<rect x="315.001" y="46.895" transform="matrix(0.9238 0.3828 -0.3828 0.9238 50.212 -118.5529)" width="16" height="39.999" />
		<path d="M248,88c-88.224,0-160,71.776-160,160s71.776,160,160,160s160-71.776,160-160S336.224,88,248,88z M248,392
				c-79.4,0-144-64.6-144-144s64.6-144,144-144s144,64.6,144,144S327.4,392,248,392z" />
		<rect x="240" width="16" height="72" />
		<rect x="62.097" y="90.096" transform="matrix(0.7071 0.7071 -0.7071 0.7071 98.0963 -40.6334)" width="71.999" height="16" />
		<rect y="240" width="72" height="16" />

		<rect x="90.091" y="361.915" transform="matrix(-0.7071 -0.7071 0.7071 -0.7071 -113.9157 748.643)" width="16" height="71.999" />
		<rect x="240" y="424" width="16" height="72" />

		<rect x="361.881" y="389.915" transform="matrix(-0.7071 -0.7071 0.7071 -0.7071 397.8562 960.6281)" width="71.999" height="16" />
		<rect x="424" y="240" width="72" height="16" />
		<rect x="389.911" y="62.091" transform="matrix(0.7071 0.7071 -0.7071 0.7071 185.9067 -252.6357)" width="16" height="71.999" />
	</svg>
	<svg version="1.1" class="moon" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px" viewBox="0 0 49.739 49.739" style="enable-background:new 0 0 49.739 49.739;" xml:space="preserve">
		<path d="M25.068,48.889c-9.173,0-18.017-5.06-22.396-13.804C-3.373,23.008,1.164,8.467,13.003,1.979l2.061-1.129l-0.615,2.268
       c-1.479,5.459-0.899,11.25,1.633,16.306c2.75,5.493,7.476,9.587,13.305,11.526c5.831,1.939,12.065,1.492,17.559-1.258v0
       c0.25-0.125,0.492-0.258,0.734-0.391l2.061-1.13l-0.585,2.252c-1.863,6.873-6.577,12.639-12.933,15.822
       C32.639,48.039,28.825,48.888,25.068,48.889z M12.002,4.936c-9.413,6.428-12.756,18.837-7.54,29.253
       c5.678,11.34,19.522,15.945,30.864,10.268c5.154-2.582,9.136-7.012,11.181-12.357c-5.632,2.427-11.882,2.702-17.752,0.748
       c-6.337-2.108-11.473-6.557-14.463-12.528C11.899,15.541,11.11,10.16,12.002,4.936z" />
	</svg>
</label>

<div class="background"></div>
!

CSS 

              
                body {
	margin: 0;
	width: 100vw;
	height: 100vh;
	display: flex;
	flex-direction: column;
	align-items: center;
	justify-content: center;
}

label {
	width: 500px;
	height: 200px;
	position: relative;
	display: block;
	background: #ebebeb;
	border-radius: 200px;
	box-shadow: inset 0px 5px 15px rgba(0, 0, 0, 0.4),
		inset 0px -5px 15px rgba(255, 255, 255, 0.4);
	cursor: pointer;
	transition: 0.3s;
	&:after {
		content: "";
		width: 180px;
		height: 180px;
		position: absolute;
		top: 10px;
		left: 10px;
		background: linear-gradient(180deg, #ffcc89, #d8860b);
		border-radius: 180px;
		box-shadow: 0px 5px 10px rgba(0, 0, 0, 0.2);
		transition: 0.3s;
	}
	svg {
		position: absolute;
		width: 120px;
		top: 40px;
		z-index: 100;
		&.sun {
			left: 40px;
			fill: #fff;
			transition: 0.3s;
		}
		&.moon {
			left: 340px;
			fill: #7e7e7e;
			transition: 0.3s;
		}
	}
}

input {
	width: 0;
	height: 0;
	visibility: hidden;
	&:checked + label {
		background: #242424;
		&:after {
			left: 490px;
			transform: translateX(-100%);
			background: linear-gradient(180deg, #777, #3a3a3a);
		}
		svg {
			&.sun {
				fill: #7e7e7e;
			}
			&.moon {
				fill: #fff;
			}
		}
		+ .background {
			background: #242424;
		}
	}
	&:active:after {
		width: 260px;
	}
}

.background {
	width: 100vw;
	height: 100vh;
	background: #fff;
	z-index: -1;
	position: absolute;
	transition: 0.3s;
}
!

JS 

              
                // Reference: https://redstapler.co/pure-css-minimal-dark-mode-toggle-button/
!
999px

Console