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CSS

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HTML 

              
                <main lang="de">
	<h1>Adventskalender</h1>
	<ul>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a href="https://www.htmhell.dev/adventcalendar/" target="adventcalendars" lang="en" style="background: white">
				<img src="https://bittersmann.de/talks/adventskalender/assets/htmhell-logo.svg" alt="HTMHell Advent Calendar" width="210" height="197" />
			</a>
		</li>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a href="https://forum.selfhtml.org/advent/2023" target="adventcalendars" lang="de" style="background: white">
				<img src="https://src.selfhtml.org/logo/selfhtml-wiki-q-nico.png" alt="SELFHTML-Adventskalender" />
			</a>
		</li>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a href="https://calendar.perfplanet.com/2023/" target="adventcalendars" lang="en" style="background: white">
				<img src="https://calendar.perfplanet.com/wp-content/themes/wpc2/wpclogo.png" alt="Web Performance Calendar" />
			</a>
		</li>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a href="https://adventjs.dev/" target="adventcalendars" lang="en" style="background: url('https://bittersmann.de/talks/adventskalender/assets/adventjs-background.jpg') center/calc(100% - 1rem)">
				<img src="https://bittersmann.de/talks/adventskalender/assets/adventjs.jpg" alt="AdventJS" />
			</a>
		</li>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a href="https://12daysofweb.dev/" target="adventcalendars" lang="en" style="background: #9eedf7">
				<img src="https://bittersmann.de/talks/adventskalender/assets/12daysofweb.png" alt="12 Days of Web" />
			</a>
		</li>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a></a>
		</li>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a href="https://www.htmhell.dev/adventcalendar/" target="adventcalendars" lang="en" style="background: white">
				<img src="https://bittersmann.de/talks/adventskalender/assets/htmhell-logo.svg" alt="HTMHell Advent Calendar" width="210" height="197" />
			</a>
		</li>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a href="https://forum.selfhtml.org/advent/2023" target="adventcalendars" lang="en" style="background: white">
				<img src="https://src.selfhtml.org/logo/selfhtml-wiki-q-nico.png" alt="SELFHTML-Adventskalender" />
			</a>
		</li>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a href="https://calendar.perfplanet.com/2023/" target="adventcalendars" lang="en" style="background: white">
				<img src="https://calendar.perfplanet.com/wp-content/themes/wpc2/wpclogo.png" alt="Web Performance Calendar" />
			</a>
		</li>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a href="https://adventjs.dev/" target="adventcalendars" lang="en" style="background: url('https://bittersmann.de/talks/adventskalender/assets/adventjs-background.jpg') center/calc(100% - 1rem)">
				<img src="https://bittersmann.de/talks/adventskalender/assets/adventjs.jpg" alt="AdventJS" />
			</a>
		</li>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a href="https://12daysofweb.dev/" target="adventcalendars" lang="en" style="background: #9eedf7">
				<img src="https://bittersmann.de/talks/adventskalender/assets/12daysofweb.png" alt="12 Days of Web" />
			</a>
		</li>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a href="https://24ways.org/" target="adventcalendars" lang="en" style="background: #e4e5eb">
				<img src="https://adrianroselli.com/wp-content/uploads/2010/12/24ways.gif" alt="24 ways" />
			</a>
		</li>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a href="https://www.24a11y.com/" target="adventcalendars" lang="en" style="background: white">
				<img src="https://bittersmann.de/talks/adventskalender/assets/24accessibility.png" alt="24 Accessibility" />
			</a>
		</li>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a href="https://www.uxmas.com/" target="adventcalendars" lang="en" style="background: #7a1560">
				<img src="https://www.uxmas.com/assets/img/uxmas-logo.svg" alt="UXmas" />
			</a>
		</li>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a href="http://webkrauts.de/serien/adventskalender/2015" target="adventcalendars" lang="de" style="background: #DDD url('https://bittersmann.de/talks/adventskalender/assets/webkrauts_background.png')">
				<img src="https://bittersmann.de/talks/adventskalender/assets/webkrauts.png" alt="Webkrauts-Adventskalender" />
			</a>
		</li>
		<li>
			<button disabled=""></button>
		</li>
		<li>
			<button disabled=""></button>
		</li>
		<li>
			<button disabled=""></button>
		</li>
		<li>
			<button disabled=""></button>
		</li>
		<li>
			<button disabled=""></button>
		</li>
		<li>
			<button disabled=""></button>
		</li>
		<li>
			<button disabled=""></button>
		</li>
		<li>
			<button disabled=""></button>
		</li>
		<li>
			<button aria-expanded="false">
				<span class="visually-hidden">Tür</span>
			</button>
			<a href="https://adrianroselli.com/2023/12/web-development-advent-calendars-for-2023.html" target="adventcalendars" lang="en" style="background: #efefef">
				<img src="https://bittersmann.de/talks/adventskalender/assets/web-advent-calendars.png" alt="Web Development Advent Calendars for 2023" />
			</a>
		</li>
	</ul>
	<footer>
		Foto:
		<a href="https://unsplash.com/de/@anniespratt?utm_content=creditCopyText&utm_medium=referral&utm_source=unsplash">Annie Spratt</a>
		auf
		<a href="https://unsplash.com/de/fotos/zwei-schwarz-weisse-vw-kafer-miniaturen-mit-ladung-oben-drauf-4s4dN2TA4bg?utm_content=creditCopyText&utm_medium=referral&utm_source=unsplash">Unsplash</a>
	</footer>
</main>
!

CSS 

              
                html {
	height: 100vh;
	background: url(https://images.unsplash.com/photo-1573648953114-ce9f86515a3d?q=80&w=3174&auto=format&fit=crop&ixlib=rb-4.0.3&ixid=M3wxMjA3fDB8MHxwaG90by1wYWdlfHx8fGVufDB8fHx8fA%3D%3D) center/cover;
	font-family: 'Luminari', 'Bodoni 72', 'Georgia', serif;
	font-variant-numeric: lining-nums proportional-nums;
	--hsl: 30deg 80% 90%;
	color: hsl(var(--hsl));
}

h1 {
	position: fixed;
	inset: auto 0 55vh;
	text-align: center;
	text-indent: -0.25em;
	color: hsl(var(--hsl) / 0.8);
	font-family: 'Luminari', 'Apple Chancery', 'Bodoni 72', 'Georgia', serif;
	font-weight: normal;
	font-size: clamp(6.66vw, 3.5rem, 10vw);
	line-height: 0;
	text-shadow: 0 0 .2em black;
}

ul {
	list-style: none;
	position: fixed;
	inset: 0;
	margin: 0;
	padding: clamp(1rem, 4vmin, 2rem);
	display: grid;
	grid-template-columns: repeat(6, 1fr);
	grid-template-rows: repeat(4, 1fr);
	gap: clamp(1rem, 4vmin, 2rem);
	
	@media (aspect-ratio < 1) {
		grid-template-columns: repeat(4, 1fr);
		grid-template-rows: repeat(6, 1fr);		
	}
}

li {
	position: relative;
	
	button {
		position: absolute;
		inset: 0;
		width: 100%;
		background-color: transparent;
		border: thin dashed hsl(var(--hsl) / 0.6);
		font: inherit;
		color: inherit;
		
		&::after {
			content: counter(list-item);
			position: absolute;
			inset: auto 0 0 auto;
			font-size: 1.2em;
			padding: 0.5rem;
		}
	}
	
	a {
		display: none;
		position: absolute;
		inset: 0;
		padding: 0.5rem;
		
		button[aria-expanded="true"] ~ & {
			background: #DDD url('https://bittersmann.de/talks/adventskalender/assets/webkrauts_background.png');
			display: flex;
			justify-content: center;
			align-items: center;
		}
	}

	:is(img, svg) {
		max-width: 100%;
		max-height: 100%;
	}
}

.site-logo__heaven {
	display: none;
}

footer {
	position: fixed;
	right: 0;
	bottom: 0;
	padding: 0.5em;
	writing-mode: vertical-rl;
	scale: -1;
	text-orientation: sideways;
	color: hsl(var(--hsl) / 0.4);
	
	a {
		color: inherit;
		text-decoration: inherit;
	}
}

/* from https://www.a11yproject.com/posts/how-to-hide-content/ */
.visually-hidden:not(:focus):not(:active) {
	clip: rect(0 0 0 0);
	clip-path: inset(50%);
	height: 1px;
	overflow: hidden;
	position: absolute;
	white-space: nowrap;
	width: 1px;
}
!

JS 

              
                // Fisher-Yates shuffle
Array.prototype.shuffle = function () {
  for (let i = this.length - 1; i > 0; i--) {
    const j = Math.floor(Math.random() * (i + 1));
    [this[i], this[j]] = [this[j], this[i]];
  }
};

// fill array with numbers from 1 to 24
const days = Array.from({length: 24}, (element, index) => index);

days.shuffle();
const liElements = document.querySelectorAll('li');
liElements.forEach((element, index) => {
	element.style.order = days[index];
});

document.documentElement.addEventListener('click', event => {
	const button = event.target.closest('button');
	if (button) {
		button.setAttribute('aria-expanded', 'true');
	}
});
!
999px

Console