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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

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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

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).

Add External Stylesheets/Pens

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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 

              
                <div id="demo">
	<p>Instructions: change your OS light/dark color-scheme.</p>
	<p>Depending on the setting, the <code>font-size</code>, <code>border-style</code>, <code>font-weight</code>, etc. will change thanks to the custom <code>--light-dark()</code> function.</p>
	<p>The text color and <code>background-color</code> also change, thanks to the built-in <code>light-dark()</code></p>
</div>

<div class="warning"><p>This is a demo for <a href="https://brm.us/css-custom-functions" target="_top">https://brm.us/css-custom-functions</a>. It relies on <a href="https://drafts.csswg.org/css-mixins/" target="_top">Custom Functions in CSS</a>. Try it in Chrome Canary with Experimental Web Platform Features enabled.</p></div>
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CSS 

              
                @function --light-dark(--light, --dark) {
	result: var(--light);
	
	@media (prefers-color-scheme: dark) {
		result: var(--dark);
	}
}

#demo {
	width: 60ch;
	text-align: center;
	text-wrap: balance;
	padding: 1em;
	
	color-scheme: light dark;
	color: light-dark(#333, #e4e4e4);
	background-color: light-dark(aliceblue, #333);
	
	font-weight: --light-dark(500, 300);
 	font-size: --light-dark(16px, 18px);
	border: 4px --light-dark(dashed, dotted) currentcolor;
	
	transition: all 0.25s ease, border-style 0.25s allow-discrete;
}

@layer layout {
	* {
		box-sizing: border-box;
	}

	:root {
		font-family: "Literata", serif;
		background: #fff;
	}

	body,
	html {
		height: 100%;
		margin: 0;
		padding: 0;
	}

	body {
		display: grid;
		place-content: safe center;
	}
}

@layer warning {
	.warning {
		padding: 1em;
		margin: 1em 0;
		border: 1px solid #ccc;
		background: rgba(255 255 205 / 0.8);
		color: black;
		text-align: center;
		position: fixed;
		bottom: 0;
		left: 1em;
		right: 1em;
	}

	.warning > :first-child {
		margin-top: 0;
	}

	.warning > :last-child {
		margin-bottom: 0;
	}

	.warning a {
		color: blue;
	}
	.warning--info {
		border: 1px solid #123456;
		background: rgb(205 230 255 / 0.8);
	}
	.warning--alarm {
		border: 1px solid red;
		background: #ff000010;
	}
}
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JS
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999px

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