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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>

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. If you need things in the <head> of the document, put that code here.

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Insecure Resource

The resource you are linking to is using the 'http' protocol, which may not work when the browser is using https.

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

To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.

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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 

              
                <div class="input-wrap">
	<div>
		<label for="js_wrap_width">width: </label>
		<input type="range" id="js_wrap_width" min="10" max="100" step="5">
	</div>
	<div>
		<label for="js_flex_type">justify-content: </label>
		<select name="js_flex_type" id="js_flex_type"></select>
	</div>
</div>


<div class="wrap">
	<p>Good</p>
	<ul class="flex-list custom">
		<li><a href="#">Home</a></li>
		<li><a href="#">Works</a></li>
		<li><a href="#">Profile</a></li>
		<li><a href="#">Links</a></li>
		<li><a href="#">Contact</a></li>
	</ul>
	<p>Bad</p>
	<ul class="flex-list origin">
		<li><a href="#">Home</a></li>
		<li><a href="#">Works</a></li>
		<li><a href="#">Profile</a></li>
		<li><a href="#">Links</a></li>
		<li><a href="#">Contact</a></li>
	</ul>
</div>
!

CSS 

              
                *, *:before, *:after {
	box-sizing: border-box;
	margin: 0;
	padding: 0;
	border: 0;
}
body {
	min-width: 600px;
	padding: .5em;
}
.wrap {
	padding: 1em;
	border: 1px solid #222;
	border-radius: 10px;
}


/* ここからflex */
.flex-list {
	position: relative;
	width: 100%;
	display: flex;
	margin-bottom: 1em;
	overflow: auto;
	align-items: center;
	flex-wrap: nowrap;
	list-style: none;
	background: #eee;
}
.flex-list li a {
	display: block;
	min-width: 100px;
	padding: .5em 1em;
	background: #333;
	color: #fff;
	text-align: center;
	text-decoration: none;
}
.flex-list li:nth-child(odd) a {
	background: #222;
}


/* ここからmarginで独自実装バージョン */
/* flex-start */
.custom.flex-start {
	justify-content: flex-start;
}

/* space-between */
.custom.space-between {
	justify-content: space-between;
}

/* space-around */
.custom.space-around li {
	margin: 0 auto;
}

/* space-evenly */
.custom.space-evenly:before,
.custom.space-evenly:after {
	content: "";
	width: 0;
	visibility: hidden;
	margin-left: auto;
}
.custom.space-evenly li {
	margin: 0 auto;
}

/* center */
.custom.center li:first-child {
	margin-left: auto;
}
.custom.center li:last-child {
	margin-right: auto;
}


/* ここからjustify-contentで指定バージョン */
.origin.flex-start {
	justify-content: flex-start;
}
.origin.space-between {
	justify-content: space-between;
}
.origin.space-around {
	justify-content: space-around;
}
.origin.space-evenly {
	justify-content: space-evenly;
}
.origin.center {
	justify-content: center;
}


/* ここからコントローラーのCSS */
.input-wrap {
	padding: 1em;
}
.input-wrap input[type=text],
.input-wrap select {
	padding: 5px;
	border: 1px solid #555;
	border-radius: 3px;
}
!

JS 

              
                
document.addEventListener("DOMContentLoaded", start);

var $inputWidth, $selectType;
var flexTypes = [
	"flex-start",
	"space-between",
	"space-around",
	"space-evenly",
	"center"
];
function start() {
	$inputWidth = document.querySelector("#js_wrap_width");
	$inputWidth.value = "100";
	$inputWidth.addEventListener("input", onInputWidth);
	$inputWidth.addEventListener("change", onInputWidth);
	
	$selectType = document.querySelector("#js_flex_type");
	$selectType.addEventListener("change", onChangeType);
	
	flexTypes.forEach(function (type) {
		$selectType.innerHTML += ""
			+"<option value='"+type+"'>"
			+type+"</option>";
	});
	
	onInputWidth();
	onChangeType();
}

function onInputWidth() {
	setWrapWidth($inputWidth.value + "%");
}

function onChangeType() {
	var $flexs = document.querySelectorAll(".flex-list");
	$flexs.forEach(function($flex) {
		flexTypes.forEach(function(type) {
			$flex.classList.remove(type);
		});
		$flex.classList.add($selectType.value);
	});
	
}

function setWrapWidth(width) {
	var $wrap = document.querySelector(".wrap");
	$wrap.style.width = width;
}
!
999px

Console