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CSS

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CSS Base

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HTML 

              
                <input type="radio" id="from-end-0" name="from" checked />
<input type="radio" id="from-end-1" name="from" />
<input type="radio" id="from-start-1" name="from" />
<input type="radio" id="from-start-0" name="from" />

<input type="radio" id="to-end-0" name="to" />
<input type="radio" id="to-end-1" name="to" />
<input type="radio" id="to-start-1" name="to" />
<input type="radio" id="to-start-0" name="to" checked />

<div id="controls">
	<div>
		<h2>From offset</h2>
		<ul>
			<li><label for="from-start-0"><code>start 0</code></label></li>
			<li><label for="from-start-1"><code>start 1</code></label></li>
			<li><label for="from-end-1"><code>end 1</code></label></li>
			<li><label for="from-end-0"><code>end 0</code></label></li>
		</ul>
	</div>
	<div>
		<h2>To offset</h2>
		<ul>
			<li><label for="to-start-0"><code>start 0</code></label></li>
			<li><label for="to-start-1"><code>start 1</code></label></li>
			<li><label for="to-end-1"><code>end 1</code></label></li>
			<li><label for="to-end-0"><code>end 0</code></label></li>
		</ul>
	</div>
</div>

<div id="browser">
	<main>
		<div class="box" data-offset="end 0"></div>
		<div class="box" data-offset="end 1"></div>
		<div class="box" data-offset="start 1"></div>
		<div class="box" data-offset="start 0"></div>
		
		<div class="box" id="from"></div>
		<div class="box" id="to"></div>
	</main>
	<aside id="scrollbar">
		<div id="thumb"></div>
	</aside>
	<div id="animation-line"></div>
</div>
<div id="base-line"></div>
		

<details>
	<summary>ℹ️</summary>
	<dialog open>
		<h1>About this Pen</h1>
		<p>ℹ️ Scroll-Linked Animations with Element-Based offsets take a somewhat quirky syntax. This tool helps you visualize how long the animation will run, based on the selected offsets.</p>
		
		<p>🚦 Valid combinations result in a green line. Invalid Combinations result in a red line.</p>
		
		<p>🤔 Want to learn more about Scroll-Linked Animations / Scroll-Timeline? Check out these two articles:</p>
		
		<ul>
			<li><a href="https://brm.us/scroll-linked-animations-pt1" target="_top">Scroll-Linked Animations with <code>@scroll-timeline</code> (Part 1) — Introduction</a></li>
			<li><a href="https://brm.us/scroll-linked-animations-pt2" target="_top">Scroll-Linked Animations with <code>@scroll-timeline</code> (Part 2) — Element-Based Offsets</a></li>
		</ul>
		
		<p>👀 More animations/visualizations: <a href="https://codepen.io/collection/AYoego" target="_top">https://codepen.io/collection/AYoego</a></p>
		
		<p>👋 Built by Bramus! — <a href="https://www.twitter.com/bramus" target="_top">@bramus</a> - <a href="https://www.bram.us/" target="_top">https://www.bram.us/</a> </p>
	</dialog>
</details>
!

CSS 

              
                :root {
	--scrollbox-border-size: 1rem;
	--scrollbox-height: 60vh;
	--scrollbox-width: 60vw;
	--box-height: 10vh;
	--box-border-size: 0.25rem;

	--thumb-size: 10vh;
	--scrollbar-width: 0.5rem;
	
	--shaded-size: 2px;
	--shaded-color: rgba(0 0 0 / 0.12);
}

html, body {
	width: 100vw;
	height: 100vh;
	background: #fff;
	margin: 0;
	padding: 0;
}

body {
	display: grid;
	place-items: center;
}

#browser {
	width: var(--scrollbox-width);
	height: var(--scrollbox-height);
	border: var(--scrollbox-border-size) solid lightblue;

	display: grid;
	grid-template-columns: 1fr var(--scrollbar-width);
	grid-template-rows: 1fr;
	grid-template-areas: "content scrollbar";

	position: relative;
	z-index: 1;
}

#browser * {
	margin: 0;
	padding: 0;
}

main {
	position: relative;
	background: transparent;
}

main::before, main::after {
	white-space: nowrap;
	display: inline-block;
	position: absolute;
	right: -1rem;
	text-transform: uppercase;
	font-size: 0.8rem;
} 

main::before {
	content: 'start edge';
	top: 0;
	transform: translate3d(calc(1em + var(--scrollbox-border-size) + 100%), calc(-50% - var(--scrollbox-border-size) / 2), 0);
}

main::after {
	content: 'end edge';
	bottom: 0;
	transform: translate3d(calc(1em + var(--scrollbox-border-size) + 100%), calc(50% + var(--scrollbox-border-size) / 2), 0);
}

#scrollbar {
	grid-area: scrollbar;
	height: 100%;
	align-self: start;

	background: #ccc;
}

#scrollbar #thumb {
	margin-top: 14vh;
	height: var(--thumb-size);
	width: 100%;
	background: #333;
	border-radius: 9999px;
}

.box {
	height: var(--box-height);
	position: absolute;
	left: 0;
	right: 0;

	border-top: var(--box-border-size) dashed #333;
}

.box[data-offset] {
	background: repeating-linear-gradient(
      -45deg,
      var(--shaded-color),
      var(--shaded-color) var(--shaded-size),
      transparent var(--shaded-size),
      transparent calc(var(--shaded-size) * 2)
    );
}

.box[data-offset="end 0"] {
	transform: translateY(calc(var(--scrollbox-height) + var(--scrollbox-border-size)));
}

.box[data-offset="end 1"] {
	transform: translateY(calc(var(--scrollbox-height) - var(--box-height) - var(--box-border-size)));
}

.box[data-offset="start 1"] {
}

.box[data-offset="start 0"] {
	transform: translateY(calc(-1 * (var(--box-height) + var(--scrollbox-border-size) + var(--box-border-size))));
}

.box::after {
	text-transform: uppercase;
	font-size: 0.8rem;
	display: inline-block;
	position: absolute;
	left: 1rem;
	top: 50%;
	display: inline-block;
	content: attr(data-offset);
	transform: translate3d(
		0,
		-50%,
		0
	);
}

input {
	display: none;
}

#base-line,
#animation-line {
	width: 0;
	height: auto;
	
	border: 0.5em solid green;
	position: absolute;
	left: calc(50% - (var(--scrollbar-width) / 2));
	top: 50%;
	bottom: 50%;
}

#base-line {
	top: 0;
	bottom: 0;
	border-color: red;
}

#from-end-0:checked ~ #browser #animation-line {
	bottom: calc(var(--scrollbox-border-size) * -1)
}

#from-end-1:checked ~ #browser #animation-line {
	bottom: calc(var(--box-height) + var(--box-border-size));
}

#from-start-1:checked ~ #browser #animation-line {
	bottom: calc(var(--scrollbox-height))
}

#from-start-0:checked ~ #browser #animation-line {
	bottom: calc(var(--scrollbox-height) + var(--scrollbox-border-size));
}

#to-start-0:checked ~ #browser #animation-line {
	top: calc((var(--box-height) + var(--scrollbox-border-size) + var(--box-border-size)) * -1)
}

#to-start-1:checked ~ #browser #animation-line {
	top: 0;
}

#to-end-1:checked ~ #browser #animation-line {
	top: calc(var(--scrollbox-height) - var(--box-height) - var(--box-border-size));
}

#to-end-0:checked ~ #browser #animation-line {
	top: calc(var(--scrollbox-height) + var(--scrollbox-border-size));
}

#from {
	color: rgba(0 0 150 / 0.5);
}
#to {
	color: rgba(150 0 150 / 0.5);
}
#from::after {
	content: 'from';
}
#to::after {
	content: 'to';
}

#from, #to {
	width: 50%;
	left: 25%;
	
	--shaded-color: lime;
	background: repeating-linear-gradient(
      45deg,
      var(--shaded-color),
      var(--shaded-color) var(--shaded-size),
      transparent var(--shaded-size),
      transparent calc(var(--shaded-size) * 2)
    );
	border-top: var(--box-border-size) solid lime;
}

#from::after, #to::after {
	left: auto;
	right: 1rem;
	color: black;
}

#from-end-0:checked ~ #browser #from,
#to-end-0:checked ~ #browser #to {
	bottom: calc((var(--box-height) + var(--scrollbox-border-size) + var(--box-border-size)) * -1);
}

#from-end-1:checked ~ #browser #from,
#to-end-1:checked ~ #browser #to {
	bottom: 0;
}

#from-start-1:checked ~ #browser #from {
	top: 0;
}

#from-start-0:checked ~ #browser #from,
#to-start-0:checked ~ #browser #to {
	top: calc((var(--box-height) + var(--scrollbox-border-size) + var(--box-border-size)) * -1);
}

/* Invalid Combinations */
#from-start-0:checked ~ #to-start-0:checked ~ #browser #animation-line,
#from-start-0:checked ~ #to-start-1:checked ~ #browser #animation-line,
#from-start-0:checked ~ #to-end-1:checked ~ #browser #animation-line,
#from-start-0:checked ~ #to-end-0:checked ~ #browser #animation-line,
#from-start-1:checked ~ #to-start-1:checked ~ #browser #animation-line,
#from-start-1:checked ~ #to-end-1:checked ~ #browser #animation-line,
#from-start-1:checked ~ #to-end-0:checked ~ #browser #animation-line,
#from-end-1:checked ~ #to-end-1:checked ~ #browser #animation-line,
#from-end-1:checked ~ #to-end-0:checked ~ #browser #animation-line,
#from-end-0:checked ~ #to-end-0:checked ~ #browser #animation-line {
	border-color: red;
}

#controls {
	width: var(--scrollbox-width);
	display: flex;
	justify-content: space-evenly;
	text-align: center;

	position: absolute;
	top: 50%;
	left: 50%;
	transform: translate3d(-50%, -50%, 0);
	z-index: 2;
}

#controls ul, #controls li {
	list-style: none;
	margin: 0;
	padding: 0;
}

#controls label {
	cursor: pointer;
}

#from-start-0:checked ~ #controls [for='from-start-0'],
#from-start-1:checked ~ #controls [for='from-start-1'],
#from-end-1:checked ~ #controls [for='from-end-1'],
#from-end-0:checked ~ #controls [for='from-end-0'],
#to-start-0:checked ~ #controls [for='to-start-0'],
#to-start-1:checked ~ #controls [for='to-start-1'],
#to-end-1:checked ~ #controls [for='to-end-1'],
#to-end-0:checked ~ #controls [for='to-end-0'] {
	font-weight: bold;
}

#controls [for]::before {
	content: '○ ';
}

#from-start-0:checked ~ #controls [for='from-start-0']::before,
#from-start-1:checked ~ #controls [for='from-start-1']::before,
#from-end-1:checked ~ #controls [for='from-end-1']::before,
#from-end-0:checked ~ #controls [for='from-end-0']::before,
#to-start-0:checked ~ #controls [for='to-start-0']::before,
#to-start-1:checked ~ #controls [for='to-start-1']::before,
#to-end-1:checked ~ #controls [for='to-end-1']::before,
#to-end-0:checked ~ #controls [for='to-end-0']::before {
	content: '● ';
}


/** Poor man's infobox */
details {
	position: fixed;
	bottom: 1em;
	right: 1em;
	font-size: clamp(1.25rem, 2vw, 2rem);
	z-index: 9999;
}
dialog[open] {
	position: fixed;
	top: 10vmin;
	right: 10vmin;
	bottom: 10vmin;
	left: 10vmin;
	width: auto;
	height: auto;

	/* Extra CSS for Safari … */
	background: #eee;
	border: 0.25em solid lightblue;
	padding: 1em;
	overflow: auto;
}
details[open] > summary::after {
	content: '';
	position: fixed;
	top: 0; right: 0; bottom: 0; left: 0;
	background: rgba(0 0 0 / 0.5);
	display: block;
}

/* Hide summary arrow …*/
details summary::-webkit-details-marker {
	display: none;
}
details summary {
	display: inline-block;
}
details summary::marker {
	display: none;
}
!

JS 

              
                import { showDialog } from 'https://codepen.io/bramus/pen/ZEqMOLz.js';
showDialog();
!
999px

Console