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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

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

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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).

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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

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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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 

              
                 <section class="u-bg--green">
	<p class="u-color--dark u-padding u-contained">
		Lorem ipsum dolor sit amet, consectetur adipiscing elit, <a href="#">sed do eiusmod tempor incididunt</a> ut labore et dolore magna aliqua. Aliquam sem fringilla ut morbi tincidunt. <a href="#">Maecenas accumsan</a> lacus vel facilisis. Posuere sollicitudin aliquam ultrices sagittis orci a scelerisque purus semper.
	</p>
</section>

<section class="u-bg--dark">
	<p class="u-color--grey u-padding u-contained">
		Lorem ipsum dolor sit amet, consectetur adipiscing elit, <a href="#">sed do eiusmod tempor incididunt</a> ut labore et dolore magna aliqua. Aliquam sem fringilla ut morbi tincidunt. <a href="#">Maecenas accumsan</a> lacus vel facilisis. Posuere sollicitudin aliquam ultrices sagittis orci a scelerisque purus semper.
	</p>
</section>

<section class="u-bg--light">
	<p class="u-color--dark u-padding u-contained">
		Lorem ipsum dolor sit amet, consectetur adipiscing elit, <a href="#">sed do eiusmod tempor incididunt</a> ut labore et dolore magna aliqua. Aliquam sem fringilla ut morbi tincidunt. <a href="#">Maecenas accumsan</a> lacus vel facilisis. Posuere sollicitudin aliquam ultrices sagittis orci a scelerisque purus semper.
	</p>
</section>

<section style="background-color: #fff; color: #000;">
	<p class="u-padding u-contained">
		What happens if you don't use utility classes to apply the colours? <a href="#">See for yourself!</a> All that you need to do is define a sensible default for your variables.
	</p>
</section>
!

CSS 

              
                html {
	// Set default colors
	--bg: #fff;
	--c: #000;
}

// Define a named list of our colors
$colors: (
	'green': #08ffc8,
	'light': #fff7f7,
	'grey': #dadada,
	'dark': #204969
);

// Loop over each color, where $n is the name (or key), $c is the color value
@each $n, $c in $colors {
	.u-color--#{$n} {
    --c: #{$c};
    color: #{$c};
	}

	.u-bg--#{$n} {
    --bg: #{$c};
    background-color: #{$c};
	}
}

// Define our default link style
a {
	// Styles for older browsers	
	text-decoration: underline;
	color: inherit;
	
	&:hover,
	&:focus,
	&:active {
		text-decoration: none;
		outline: .0625em solid currentColor;
		outline-offset: .0625em;
	}
	
	&:active {
    outline-width: .125em;
	}

	@supports (--a: b) {
		--space: .125em;
		--link-color: currentColor;
		--bg-size: var(--space);
		--shadow-size: 0;
    
    padding: var(--space); // Inline elements won't affect vertical rhythm, so we don't need to specify each direction
		
		color: var(--link-color); // Use the variable for our color
		text-decoration: none; // Remove the default underline
		
    border-radius: var(--space); // Make it a tiny bit fancier ✨
    background-image: linear-gradient(var(--c, currentColor), var(--c, currentColor));
    background-repeat: repeat-x;
    background-position: 0 100%;
    background-size: 100% var(--bg-size);
    box-shadow: 0 0 0 var(--shadow-size, 0) var(--c, currentColor); // Used in the :active state
    box-decoration-break: clone; // Ensure the styles repeat on links spaning multiple lines
    
    transition-property: color, background-size, box-shadow;
    transition-duration: 150ms;
    transition-timing-function: ease-in-out;
    will-change: color, background-size, box-shadow;
	  
		&:link,
    &:visited {
			--link-color: var(--c, currentColor); // Use --c, or fallback to currentColor
		}
    
    &:hover,
    &:focus,
    &:active {
      --bg-size: 100%;
      --link-color: var(--bg);
			outline: none; // Remove the fallback style
    }
    
    &:active {
      --shadow-size: var(--space); // Define the box-shadow size
    }
	}
}

// Other utility classes
.u-contained {
  max-width: 60ch;
  margin: 0 auto;
}

.u-padding { padding: 1rem; }

// Resets…
* { 
  margin: 0;
  line-height: 1.4em;
  box-sizing: border-box;
}

html,
body {
  height: 100%;
}
!

JS
!
999px

Console