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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).

Add External Stylesheets/Pens

Any URLs added here will be added as <link>s in order, and before the CSS in the editor. You can use the CSS from another Pen by using its URL and the proper URL extension.

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You can apply CSS to your Pen from any stylesheet on the web. Just put a URL to it here and we'll apply it, in the order you have them, before the CSS in the Pen itself.

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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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You can apply a script from anywhere on the web to your Pen. Just put a URL to it here and we'll add it, in the order you have them, before the JavaScript in the Pen itself.

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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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 

              
                - let a = .65; //- desired alpha
- let u = Math.min(a, 1 + a*a - 2*a); //- base (unit) interval
- let n = Math.ceil(1/u); //- number of intervals
//- alpha point values, most mapped to desired alpha save for 0 and 1
- let v = new Array(n + 1).fill(0).map((_, i) => i*(n - i) ? a : i/n)

mixin text(cls)
	p(class=cls): span: span Socks go missing, vases fall,
		br
		| Claws on curtains, they climb the wall.
		br
		| One swift pounce, a sudden bite,
		br
		| Then they purr like all is right.

body(style=`--a: ${a}`)
	svg(width='0' height='0' aria-hidden='true')
		filter#alpha(color-interpolation-filters='sRGB')
			feComponentTransfer
				feFuncA(type='table' tableValues=v.join(' '))
	
	h2 Problem 😵
	+text
	h2 SVG filter fix 😃
	+text('fix')
	
	form
		label(for='a') alpha
		input#a(type='range' value=a*100 min='5' max='85')
	
	pre
		span.tkn--punc <
		span.tkn--elem feFuncA
		|  
		span.tkn--attr type
		span.tkn--oper =
		span.tkn--aval 'table'
		|  
		span.tkn--attr tableValues
		span.tkn--oper =
		span.tkn--aval#v '
			- v.forEach((c, i) => {
				if i
					| 
				span.tkn--ival #{c.toString().replace('0.', '.')}
			- })
			| '
		span.tkn--punc />
!

CSS 

              
                /* relevant code */
p {
	/* prevent blending with page backdrop */
	isolation: isolate;
	
	&.fix { filter: url(#alpha) }
}

p > span {
	padding: .5em;
	border-radius: 5px;
	/* for the fix, background must be semi-transparent, 
	 * but the exact final alpha value is set by filter */
	background: rgb(9.8% 9.8% 43.9%/ var(--a));
	color: rgb(100% 64.7% 0%); /* opaque text */
	/* Safari unfortunately still needs the prefix */
	-webkit-box-decoration-break: clone;
	box-decoration-break: clone;
	
	/* inner span fixes background-letter overlap */
	/* only on the inner one because Firefox */
	/* for dark text on light background, change to darken */
	span { mix-blend-mode: lighten }
}

/* layout, prettifying/ secondary stuff */
$thumb-d: 1.5em;
$thumb-r: .5*$thumb-d;

$track-h: 6px;
$track-r: .5*$track-h;

@mixin thumb($f: 0) {
	@if $f > 0 {
		margin-top: calc(#{$track-r} + -1*#{$thumb-r})
	}
	
	border: solid 2px 
		RGB(from #040404 r g b/ calc(1 - var(--hov)));
	width: $thumb-d;
	height: $thumb-d;
	border-radius: 50%;
	scale: calc(1 - .25*(1 - var(--hov)));
	background: dodgerblue;
	filter: Saturate(var(--hov));
	cursor: ew-resize;
	transition: .35s ease-out
}

@mixin track() {
	height: $track-h;
	border-radius: $track-r;
	background: grey;
}

html, body, form { display: grid }

html { min-height: 100% }

body {
	grid-gap: .5em;
	grid-template-rows: repeat(5, auto) 4lh;
	align-content: center;
	place-items: start center;
	background: 
		url(https://images.unsplash.com/photo-1491485880348-85d48a9e5312?w=2070) 
		50%/ cover #333;
	background-blend-mode: multiply;
	font: clamp(.75em, 5vmin, 2em)/ 1.25 rokkitt, serif
}

/* functionally the same as a style, 
 * take it out of document flow */
svg[height='0'][aria-hidden='true'] { position: fixed }

h2 {
	margin: 0;
	padding: .25em .375em .125em;
	color: tomato;
	background: #000c;
	
	&:has(+ p.fix) { color: yellowgreen }
}

p, pre {
	box-sizing: border-box;
	padding: .5em
}

p {
	justify-self: center;
	margin: 0;
	width: Min(100%, 30em);
	text-align: center
}

form { place-items: center }

label { color: #ededed }

[type='range'] {
	&, &::-webkit-slider-thumb { appearance: none }
	
	--hov: 0;
	width: 20em;
	max-width: 100%;
	background: none;
	cursor: pointer;
	
	&::-webkit-slider-runnable-track { @include track }
	&::-moz-range-track { @include track }
	
	&::-webkit-slider-thumb { @include thumb(1) }
	&::-moz-range-thumb { @include thumb() }
	
	&:focus { outline: none }
	
	&:focus, &:hover { --hov: 1 }
}

pre {
	overflow-x: scroll;
	scrollbar-width: thin;
	scrollbar-color: var(--cs, gold) #0000;
	border: solid 2px #040404;
	width: Min(100%, 40em);
	border-radius: 5px;
	background: #121212;
	color: #ededed;
	font: Max(.75rem, .5em) monospace;
	
	output { display: inline }
	
	@media (min-width: 560px) { --cs: #0000 }
}

[class^='tkn'] {
	&:nth-child(2 of .tkn--attr)::before {
		content: '\A          '
	}
	
	&:not(:first-child):nth-child(11n + 1 of .tkn--ival)::before {
		content: '\A                      '
	}
}

.tkn {
	&--elem { color: #ffcc66 }
	&--attr { color: #66ccff }
	
	&--aval { color: #99cc33 }
	&--ival { color: #ff6699 }
}
!

JS 

              
                const _f = document.querySelector('feFuncA'), 
			_v = document.getElementById('v')

addEventListener('input', e => {
	let _t = e.target, 
			a = +(_t.value*.01).toFixed(2), 
			u = Math.min(a, 1 + a*a - 2*a), 
			n = Math.ceil(1/u), 
			v = new Array(n + 1).fill(0).map((_, i) => i*(n - i) ? a : i/n);
	
	document.body.style.setProperty('--a', a);
	_f.setAttribute('tableValues', v.join(' '));
	_v.innerHTML = `'${v.map(c => `<span class='tkn--ival'>${c.toString().replace('0.', '.')}</span>`).join(' ')}'`;
})
!
999px

Console