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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>

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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

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JavaScript

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

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Babel includes JSX processing.

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HTML 

              
                		<header>
			<h1>View Transitions Demo: Accordion</h1>
		</header>
		<main>
			<div class="accordion">
				<div class="item" id="item-1">
					<input type="radio" name="accordion" value="unblur" id="unblur" checked />
					<label for="unblur">
						<span class="title">Photo Unblur fixes your pics, even the old ones.</span>
						<span class="marker">&rsaquo;</span>
					</label>
					<div>
						<h2>Photo Unblur fixes your pics, even the old ones.</h2>
						<p>With a tap, sharpen and brighten picture-imperfect photos – even ones from non-Pixel phones. #FixedOnPixel</p>
					</div>
				</div>
				<div class="item" id="item-2">
					<input type="radio" name="accordion" value="magic-eraser" id="magic-eraser" />
					<label for="magic-eraser">
						<span class="title">Easily remove distractions in photos with Magic Eraser.</span>
						<span class="marker">&rsaquo;</span>
					</label>
					<div>
						<h2>Easily remove distractions in photos with Magic Eraser.</h2>
						<p>Make unwanted objects or photobombers disappear. Or change the color and brightness so an object blends right in. #FixedOnPixel</p>
					</div>
				</div>
				<div class="item" id="item-3">
					<input type="radio" name="accordion" value="night-sight" id="night-sight" />
					<label for="night-sight">
						<span class="title">Details pop out in the dark with Night Sight.</span>
						<span class="marker">&rsaquo;</span>
					</label>
					<div>
						<h2>Details pop out in the dark with Night Sight.</h2>
						<p>Now it’s easier to create rich, vibrant photos when it’s dark – city lights, portraits in low light, and starry skies with astrophotography.</p>
					</div>
				</div>
				<div class="item" id="item-4">
					<input type="radio" name="accordion" value="super-res-zoom" id="super-res-zoom" />
					<label for="super-res-zoom">
						<span class="title">Get amazing closeups with Super Res Zoom.</span>
						<span class="marker">&rsaquo;</span>
					</label>
					<div>
						<h2>Get amazing closeups with Super Res Zoom.</h2>
						<p>The Pixel 7a camera includes Super Res Zoom. So you can get up close without an extra telephoto lens.</p>
					</div>
				</div>
				<div class="item" id="item-5">
					<input type="radio" name="accordion" value="long-exposure" id="long-exposure" />
					<label for="long-exposure">
						<span class="title">Create energetic photos with Long Exposure.</span>
						<span class="marker">&rsaquo;</span>
					</label>
					<div>
						<h2>Create energetic photos with Long Exposure.</h2>
						<p>Add motion, texture, and artful blurs – including light trails – to waterfalls, crowds, city streets, and more. All without a tripod.</p>
					</div>
				</div>
			</div>
		</main>
		<footer>
			<p>Example URL: <a href="https://store.google.com/us/product/pixel_7a?hl=en-US">Pixel 7a – Google Store</a></p>
		</footer>
!

CSS 

              
                @layer reset, layout, transitions;

@layer transitions {
	:root {
		view-transition-name: none;
	}

	#item-1 {
		view-transition-name: item-1;
		.marker {
			view-transition-name: marker-1;
		}
		&:has(input:checked) {
			& h2 {
				view-transition-name: title-1;
			}
		}
		&:not(.item:has(input:checked)) {
			& .title {
				view-transition-name: title-1;
			}
		}
	}

	#item-2 {
		view-transition-name: item-2;
		.marker {
			view-transition-name: marker-2;
		}
		&:has(input:checked) {
			& h2 {
				view-transition-name: title-2;
			}
		}
		&:not(.item:has(input:checked)) {
			& .title {
				view-transition-name: title-2;
			}
		}
	}

	#item-3 {
		view-transition-name: item-3;
		.marker {
			view-transition-name: marker-3;
		}
		&:has(input:checked) {
			& h2 {
				view-transition-name: title-3;
			}
		}
		&:not(.item:has(input:checked)) {
			& .title {
				view-transition-name: title-3;
			}
		}
	}

	#item-4 {
		view-transition-name: item-4;
		.marker {
			view-transition-name: marker-4;
		}
		&:has(input:checked) {
			& h2 {
				view-transition-name: title-4;
			}
		}
		&:not(.item:has(input:checked)) {
			& .title {
				view-transition-name: title-4;
			}
		}
	}

	#item-5 {
		view-transition-name: item-5;
		.marker {
			view-transition-name: marker-5;
		}
		&:has(input:checked) {
			& h2 {
				view-transition-name: title-5;
			}
		}
		&:not(.item:has(input:checked)) {
			& .title {
				view-transition-name: title-5;
			}
		}
	}

	@keyframes rotate-marker {
		from {
			rotate: 90deg;
		}
		to {
			rotate: 270deg;
			opacity: 0;
		}
	}
	::view-transition-group(*) {
		animation-duration: 0.5s;
	}
	::view-transition-group(item-1),
	::view-transition-group(item-2),
	::view-transition-group(item-3),
	::view-transition-group(item-4),
	::view-transition-group(item-5) {
		overflow: hidden;
	}

	::view-transition-old(marker-1):only-child,
	::view-transition-old(marker-2):only-child,
	::view-transition-old(marker-3):only-child,
	::view-transition-old(marker-4):only-child,
	::view-transition-old(marker-5):only-child {
		animation-name: rotate-marker;
	}
	::view-transition-new(marker-1):only-child,
	::view-transition-new(marker-2):only-child,
	::view-transition-new(marker-3):only-child,
	::view-transition-new(marker-4):only-child,
	::view-transition-new(marker-5):only-child {
		animation-name: rotate-marker;
		animation-direction: reverse;
	}
}

@layer layout {
	@layer general {
		body {
			background: #efeae6;
			font-family: 'Google Sans', Roboto, Arial, sans-serif;
			padding-top: 3rem;
		}
		header,
		footer {
			text-align: center;
		}
		main {
			margin: 2em auto;
			width: 90%;
			max-width: 30em;
		}
	}
	@layer accordion {
		.accordion {
			display: grid;
			grid-auto-flow: row;
			gap: 0.5em;
			border-radius: 1em;
			overflow: hidden;
			font-size: 1.25em;
		}

		.item {
			background: #f8f6f2;

			display: flex;
			flex-direction: column;

			& input {
				position: absolute;
				opacity: 0;
			}

			& label {
				flex: 1;
				cursor: pointer;
				padding: 1em;

				display: flex;
				flex-direction: row;
				gap: 1em;
				justify-content: space-between;
			}

			.marker {
				display: grid;
				place-content: center;
				height: 1.5rem;
				aspect-ratio: 1;
				border-radius: 50%;
				background-color: #fff;
				color: #5f6368;

				font-weight: 700;
				font-size: 1.5em;
				line-height: 1;
				rotate: 90deg;
			}

			& div {
				padding: 1em;

				:first-child {
					margin-top: 0;
				}
				:last-child {
					margin-bottom: 0;
				}
			}
		}

		/* Show/Hide label/content based on input being checked or not */
		.item:has(input:checked) {
			& label {
				display: none;
			}
		}
		.item:not(.item:has(input:checked)) {
			& div {
				display: none;
			}
		}
	}
}

@layer reset {
	html,
	body {
		height: 100%;
		margin: 0;
		padding: 0;
	}
}
!

JS 

              
                document.querySelectorAll('.item input').forEach(($input) => {
	// @note: we listen on click because that happens *before* a change event
	$input.addEventListener('click', async (e) => {
		if (!document.startViewTransition) return;

		e.preventDefault();

		const t = document.startViewTransition(() => {
			e.target.checked = true;
		});

		await t.finished;
		console.log('done');
	});
});
!
999px

Console