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HTML

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CSS

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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

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JavaScript

About JavaScript Preprocessors

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 

              
                <main>
	<input type=checkbox id=un> <label for=un>un</label>
	<input type=checkbox id=deux checked> <label for=deux>deux</label>
	<input type=checkbox id=trois checked> <label for=trois>trois</label>
	<input type=checkbox id=quatre> <label for=quatre>quatre</label>
	<input type=checkbox id=cinq> <label for=cinq>cinq</label>
</main>
!

CSS 

              
                input{
	display: grid;
	background: #ddd;
	border-radius: 20px;
	width: 60px; height: 40px;
	appearance: none; -webkit-appearance: none; 
	cursor: pointer;
	border: 0;
	margin: 0;
}
input:not(:first-of-type)::before{
	content: '';	
	/* move up a row */
	transform: translatey(-60px); 
	pointer-events: none;
}
/* when checked */
input:checked + * + input::before,
input:last-of-type:checked{
	border-radius: 20px;
	background: blue;
}
input:checked + * + input + * + input::before{
	border-top-left-radius: unset !important;
	border-top-right-radius: unset !important;
}
input:checked::before{
	border-bottom-left-radius: unset !important;
	border-bottom-right-radius: unset !important;
}
input:nth-of-type(4):checked + * + input:checked {
	border-top-left-radius: unset ;
	border-top-right-radius: unset ;
}
label{
	font: 50px Girassol;
	color: lime;
	text-shadow: 2px 2px 0 blue;
	cursor: pointer;
}
main{
	display: grid;
	grid:  repeat(5, 60px) / 90px 1fr;
	align-items: center;
	width: max-content;
}
body{
	display: grid;
	height: 100vh;
	align-items: center;
	justify-items: center;
	user-select: none; -webkit-user-select:  none;
	-webkit-tap-highlight-color: transparent;
}

/*

First off, my code is for heuristic purposes only. When using this method, ensure you've a solid layout that doesn't displace the pseudo-elements from top of the preceding boxes in all instances. Also, remember to use a layout that allows you to use the next element selector. The unset border values override the set border values only in the checked states — That's why the unchecked boxes' borders are unaffected by the checked boxes. You can also try this with other suitable shapes.The asterik character used in the code is the label between the boxes. 

*/
!

JS
!
999px

Console