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HTML

About HTML Preprocessors

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

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.

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.

!
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-.

We offer two popular choices: Autoprefixer (which processes your CSS server-side) and -prefix-free (which applies prefixes via a script, client-side).

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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 

              
                <header>
	<p>Hamburger</p>
	<nav>
		<button aria-controls="nav-collapsible" hidden="">Menu</button>
		<div id="nav-collapsible">
			<ul>
				<li><a href="">Hamburg-Mitte</a></li>
				<li><a href="">Altona</a></li>
				<li><a href="">Eimsbüttel</a></li>
				<li><a href="">Hamburg-Nord</a></li>
				<li><a href="">Wandsbek</a></li>
				<li><a href="">Bergedorf</a></li>
				<li><a href="">Harburg</a></li>
			</ul>
		</div>
	</nav>
</header>
<main id="main">
	<p>Claudius Ptolemy (2nd century AD) reported the first name for the vicinity as Treva.</p>
	<p>The name Hamburg comes from the first permanent building on the site, a castle which the Emperor Charlemagne ordered constructed in AD 808. It rose on rocky terrain in a marsh between the River Alster and the River Elbe as a defence against Slavic incursion, and acquired the name Hammaburg, burg meaning castle or fort. The origin of the Hamma term remains uncertain, as does the exact location of the castle.</p>
	<footer>(Source: Wikipedia)</footer>
</main>
!

CSS 

              
                * { box-sizing: border-box }

html
{
	background: black;
}

body
{
	margin: auto;
	max-width: 24em;
	min-height: 100vh;
	background: white;
	position: relative;
	font-family: Source Sans Pro, Open Sans, Calibri, sans-serif;
}

header
{
	padding: 1em;
	display: flex;
	flex-flow: row wrap;
	justify-content: space-between;
	align-items: center;
	background: #da121a;
	color: white;
}

button[aria-controls="nav-collapsible"]
{
	font: inherit;
	color: currentcolor;
	border: thin solid;
	border-radius: 0.25em;
	padding: 0.5em 2em;
	background: transparent url(https://upload.wikimedia.org/wikipedia/commons/5/5d/DEU_Hamburg_COA.svg) 0.5em center / auto 80% no-repeat;
	position: relative;
}

button[aria-controls="nav-collapsible"]::after
{
	content: '▶︎';
	position: absolute;
	right: 1em;
	bottom: 0.9em;
	font-size: 0.7em;
}

button[aria-controls="nav-collapsible"][aria-expanded="true"]::after
{
	content: '▼'
}

#nav-collapsible
{
	position: absolute;
	top: var(--page-header-height);
	right: 0;
	bottom: 0;
	left: 0;
	background: hsl(0deg 0% 95%);
	color: black;
	transition: 0.1s transform ease-in-out;
	transform-origin: right;
}

#nav-collapsible[hidden]
{
	display: block !important;
	transform: scaleX(0);
}

nav ul
{
	list-style: none;
  list-style: url("data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg'/%3E"); /* Safari hack, see https://www.scottohara.me/blog/2019/01/12/lists-and-safari.html */
	margin: 0;
	padding: 0;
}

nav li:not(:last-child)
{
	border-bottom: thin solid silver;
}

nav a
{
	display: block;
	padding: 0.5em 1em;
	color: currentcolor;
	text-decoration: none;
}

:focus
{
	background-color: black !important;
	color: white !important;
}

main
{
	padding: 1em;
	max-width: 42em;
}
!

JS 

              
                const collapsibleElement = document.querySelector('#nav-collapsible');
collapsibleElement.controlElement = document.querySelector('[aria-controls="nav-collapsible"]');

collapsibleElement.toggle = (event, force) => {
	const isToBeVisible = force !== undefined ? force : collapsibleElement.hidden;
	collapsibleElement.hidden = !isToBeVisible;
	collapsibleElement.setAttribute('aria-hidden', !isToBeVisible);
	collapsibleElement.controlElement.setAttribute('aria-expanded', isToBeVisible);
	if (!isToBeVisible)
	{
		//collapsibleElement.controlElement.focus();
	}
}

collapsibleElement.close = event => collapsibleElement.toggle(event, false);
collapsibleElement.open = event => collapsibleElement.toggle(event, true);

collapsibleElement.close();

collapsibleElement.controlElement.hidden = false;

collapsibleElement.controlElement.addEventListener('click', collapsibleElement.toggle);

document.documentElement.addEventListener('click', event => {
	if (event.target !== collapsibleElement.controlElement && !event.target.closest('#nav-collapsible'))
	{
		collapsibleElement.close();
	}
});

document.documentElement.addEventListener('keyup', event => {
	if (event.code === 'Escape')
	{
		collapsibleElement.close();
		collapsibleElement.controlElement.focus();
	}
});

const pageHeaderElement = document.querySelector('header');
	
pageHeaderElement.setHeightCustomProperty = () => {
	if (window.CSS && CSS.supports && CSS.supports('--page-header-height: 0'))
	{
		pageHeaderElement.style.setProperty('--page-header-height', `${pageHeaderElement.offsetHeight}px`);
	}
	else
	{
		document.querySelector('#nav-collapsible').style.setProperty('top', `${pageHeaderElement.offsetHeight}px`);
	}
};

window.addEventListener('load', pageHeaderElement.setHeightCustomProperty);
window.addEventListener('resize', pageHeaderElement.setHeightCustomProperty);
!
999px

Console