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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).

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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About External Resources

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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+ add another resource

JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

Add External Scripts/Pens

Any URL's added here will be added as <script>s in order, and run before the JavaScript in the editor. You can use the URL of any other Pen and it will include the JavaScript from that Pen.

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About External Resources

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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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 

              
                <input type="checkbox" />
<div></div>

<footer>
	<p>Extra Demo for <a href="https://brm.us/animated-gradient-border" target="_top">https://brm.us/animated-gradient-border</a>,<br />including a fallback for non-Houdini browsers.</p>
</footer>
!

CSS 

              
                div {
	width: 50vmin;
	height: 50vmin;
	border: 10vmin solid;
	border-image: conic-gradient(from 0deg, red, yellow, lime, aqua, blue, magenta, red) 1;
	animation: 10s rotate linear infinite;
}


/* Houdini Browsers: Animate the gradient using Houdini Magic */
/* As we can't use @supports for @property (see https://github.com/w3c/csswg-drafts/issues/2463) we're using another way to target browsers that speak Houdini. */
@supports (background: paint(worklet)) {
	@property --angle {
		syntax: '<angle>';
		initial-value: 0deg;
		inherits: false;
	}

	div {
		--angle: 0deg;
		border-image: conic-gradient(from var(--angle), red, yellow, lime, aqua, blue, magenta, red) 1;
	}

	@keyframes rotate {
		to {
			--angle: 360deg;
		}
	}
}

/* Non-Houdini Browsers: Manually adjust the border-image frame by frame … */
/* Code was generate using the little JS snippet included with this pen */
@supports not (background: paint(worklet)) {
	@keyframes rotate {
	  0% { border-image: conic-gradient(from 0.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  1% { border-image: conic-gradient(from 3.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  2% { border-image: conic-gradient(from 7.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  3% { border-image: conic-gradient(from 10.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  4% { border-image: conic-gradient(from 14.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  5% { border-image: conic-gradient(from 18.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  6% { border-image: conic-gradient(from 21.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  7% { border-image: conic-gradient(from 25.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  8% { border-image: conic-gradient(from 28.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  9% { border-image: conic-gradient(from 32.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  10% { border-image: conic-gradient(from 36.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  11% { border-image: conic-gradient(from 39.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  12% { border-image: conic-gradient(from 43.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  13% { border-image: conic-gradient(from 46.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  14% { border-image: conic-gradient(from 50.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  15% { border-image: conic-gradient(from 54.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  16% { border-image: conic-gradient(from 57.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  17% { border-image: conic-gradient(from 61.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  18% { border-image: conic-gradient(from 64.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  19% { border-image: conic-gradient(from 68.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  20% { border-image: conic-gradient(from 72.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  21% { border-image: conic-gradient(from 75.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  22% { border-image: conic-gradient(from 79.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  23% { border-image: conic-gradient(from 82.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  24% { border-image: conic-gradient(from 86.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  25% { border-image: conic-gradient(from 90.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  26% { border-image: conic-gradient(from 93.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  27% { border-image: conic-gradient(from 97.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  28% { border-image: conic-gradient(from 100.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  29% { border-image: conic-gradient(from 104.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  30% { border-image: conic-gradient(from 108.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  31% { border-image: conic-gradient(from 111.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  32% { border-image: conic-gradient(from 115.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  33% { border-image: conic-gradient(from 118.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  34% { border-image: conic-gradient(from 122.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  35% { border-image: conic-gradient(from 126.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  36% { border-image: conic-gradient(from 129.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  37% { border-image: conic-gradient(from 133.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  38% { border-image: conic-gradient(from 136.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  39% { border-image: conic-gradient(from 140.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  40% { border-image: conic-gradient(from 144.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  41% { border-image: conic-gradient(from 147.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  42% { border-image: conic-gradient(from 151.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  43% { border-image: conic-gradient(from 154.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  44% { border-image: conic-gradient(from 158.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  45% { border-image: conic-gradient(from 162.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  46% { border-image: conic-gradient(from 165.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  47% { border-image: conic-gradient(from 169.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  48% { border-image: conic-gradient(from 172.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  49% { border-image: conic-gradient(from 176.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  50% { border-image: conic-gradient(from 180.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  51% { border-image: conic-gradient(from 183.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  52% { border-image: conic-gradient(from 187.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  53% { border-image: conic-gradient(from 190.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  54% { border-image: conic-gradient(from 194.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  55% { border-image: conic-gradient(from 198.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  56% { border-image: conic-gradient(from 201.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  57% { border-image: conic-gradient(from 205.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  58% { border-image: conic-gradient(from 208.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  59% { border-image: conic-gradient(from 212.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  60% { border-image: conic-gradient(from 216.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  61% { border-image: conic-gradient(from 219.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  62% { border-image: conic-gradient(from 223.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  63% { border-image: conic-gradient(from 226.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  64% { border-image: conic-gradient(from 230.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  65% { border-image: conic-gradient(from 234.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  66% { border-image: conic-gradient(from 237.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  67% { border-image: conic-gradient(from 241.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  68% { border-image: conic-gradient(from 244.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  69% { border-image: conic-gradient(from 248.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  70% { border-image: conic-gradient(from 252.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  71% { border-image: conic-gradient(from 255.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  72% { border-image: conic-gradient(from 259.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  73% { border-image: conic-gradient(from 262.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  74% { border-image: conic-gradient(from 266.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  75% { border-image: conic-gradient(from 270.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  76% { border-image: conic-gradient(from 273.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  77% { border-image: conic-gradient(from 277.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  78% { border-image: conic-gradient(from 280.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  79% { border-image: conic-gradient(from 284.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  80% { border-image: conic-gradient(from 288.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  81% { border-image: conic-gradient(from 291.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  82% { border-image: conic-gradient(from 295.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  83% { border-image: conic-gradient(from 298.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  84% { border-image: conic-gradient(from 302.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  85% { border-image: conic-gradient(from 306.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  86% { border-image: conic-gradient(from 309.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  87% { border-image: conic-gradient(from 313.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  88% { border-image: conic-gradient(from 316.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  89% { border-image: conic-gradient(from 320.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  90% { border-image: conic-gradient(from 324.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  91% { border-image: conic-gradient(from 327.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  92% { border-image: conic-gradient(from 331.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  93% { border-image: conic-gradient(from 334.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  94% { border-image: conic-gradient(from 338.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  95% { border-image: conic-gradient(from 342.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  96% { border-image: conic-gradient(from 345.60deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  97% { border-image: conic-gradient(from 349.20deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  98% { border-image: conic-gradient(from 352.80deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  99% { border-image: conic-gradient(from 356.40deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	  100% { border-image: conic-gradient(from 360.00deg, red, yellow, lime, aqua, blue, magenta, red) 1; }
	}
}















input[type="checkbox"] {
	position: fixed;
	top: 1em;
	left: 1em;
}

input[type="checkbox"]:after {
	content: 'Toggle Fill';
	white-space: nowrap;
	padding-left: 1.5em;
}

input[type="checkbox"]:checked + div {
	border-image-slice: 1 fill;
}

body {
	margin: 0;
	padding: 0;
	height: 100vh;
	width: 100vw;
	display: grid;
	place-items: center;
	background: #fff;
}

footer {
	text-align: center;
	font-style: italic;
}
!

JS 

              
                // @note: This is a little helper to quickly generate the keyframes.
// It not active, but left here for debugging purposes …
const generateCSS = () => {
	let css = '@keyframes rotate {';
	for (let i = 0; i <= 100; i++) {
		css = `${css}
  ${i}% { border-image: conic-gradient(from ${(360 * i / 100).toFixed(2)}deg, red, yellow, lime, aqua, blue, magenta, red) 1; }`;
	}

	return `${css}
	}`;
}

const injectCSS = (css) => {
	const style = document.createElement('style');
	style.innerHTML = css;
	document.head.appendChild(style);
}

// console.log(generateCSS());
!
999px

Console