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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.

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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-.

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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If the script you link to has the file extension of a preprocessor, we'll attempt to process it before applying.

You can also link to another Pen here, and we'll pull the JavaScript from that Pen and include it. If it's using a matching preprocessor, we'll combine the code before preprocessing, so you can use the linked Pen as a true dependency.

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Packages

Add Packages

Search for and use JavaScript packages from npm here. By selecting a package, an import statement will be added to the top of the JavaScript editor for this package.

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About Packages

Using packages here is powered by esm.sh, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ESM usage.

All packages are different, so refer to their docs for how they work.

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HTML 

              
                ul
	li Lorem ipsum dolor sit amet, consectetur adipiscing elit. Bonum incolumis acies: misera caecitas. Quas enim kakaw Graeci appellant, vitia malo quam malitias nominare. Idem iste, inquam, de voluptate quid sentit? Recte dicis; Quaesita enim virtus est, non quae relinqueret naturam, sed quae tueretur. Haec dicuntur inconstantissime.
	li Lorem ipsum dolor sit amet, consectetur adipiscing elit. In ipsa enim parum magna vis inest, ut quam optime se habere possit, si nulla cultura adhibeatur. Duo Reges: constructio interrete. Desideraret enim valitudinem, vacuitatem doloris, appeteret etiam conservationem sui earumque rerum custodiam finemque, sibi constitueret secundum naturam vivere. Atque adhuc ea dixi, causa cur Zenoni non fuisset, quam ob rem a superiorum auctoritate discederet. Nam his libris eum malo quam reliquo ornatu villae delectari. Utrum igitur percurri omnem Epicuri disciplinam placet an de una voluptate quaeri, de qua omne certamen est? Quid turpius quam sapientis vitam ex insipientium sermone pendere? Quae enim dici Latine posse non arbitrabar, ea dicta sunt a te verbis aptis nec minus plane quam dicuntur a Graecis.
	li Lorem ipsum dolor sit amet, consectetur adipiscing elit. Poterat autem inpune; Cur ipse Pythagoras et Aegyptum lustravit et Persarum magos adiit? De ingenio eius in his disputationibus, non de moribus quaeritur. Haec qui audierit, ut ridere non curet, discedet tamen nihilo firmior ad dolorem ferendum, quam venerat. Non quaero, quid dicat, sed quid convenienter possit rationi et sententiae suae dicere. Hoc autem loco tantum explicemus haec honesta, quae dico, praeterquam quod nosmet ipsos diligamus, praeterea suapte natura per se esse expetenda. Duo Reges: constructio interrete. In qua si nihil est praeter rationem, sit in una virtute finis bonorum; Is ita vivebat, ut nulla tam exquisita posset inveniri voluptas, qua non abundaret. Sit ista in Graecorum levitate perversitas, qui maledictis insectantur eos, a quibus de veritate dissentiunt.
	li Lorem ipsum dolor sit amet, consectetur adipiscing elit. Ego vero isti, inquam, permitto. Tuo vero id quidem, inquam, arbitratu. Istam voluptatem, inquit, Epicurus ignorat? Cur post Tarentum ad Archytam?
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CSS 

              
                
//What I would like to be able to do (not real CSS)
li:nth-child(3) {
	flex-clear: clear;
}

ul {
	// What I have to do instead :(
	height: 484px;

	// flex-wrap must be set to `wrap` for flex-clear to work
	flex-wrap: wrap; 
	display: flex;
	
	width: 1000px;
	border: 2px solid red;
	flex-direction: column;
	padding: 0;
	margin: 20px;
}

li {
	list-style: none;
	background: lightblue;
	padding: 20px;
	border: 10px solid transparent;
	background-clip: padding-box;
	width: 50%;
	flex-grow: 1;
	clear: none;
}

* {
	box-sizing: border-box;
}
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JS
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999px

Console