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HTML

              
                <figure>
  <p class="image-compare" role="img" tabindex="0" aria-label="When comparing the photos, it becomes noticable that the black-and-white image lacks color." aria-description="This image is actually a set of two images, and when the set is focused, the two contained images are alternated between.">
    <span>
      <img src="//placekitten.com/400/300" alt="Random cat picture, with color taken away.">
    </span>
    <img src="//placekitten.com/400/300" alt="Unaltered random cat picture.">
  </p>
  <figcaption>Comparing the black-and-white version of a photo with its colored counterpart.</figcaption>
</figure>

              
            
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CSS

              
                .image-compare {
	--splitter-color: hotpink;
	--splitter-size: 0.125rem;
	--expand: 0.875rem;
	--handle-size: calc(var(--expand) + var(--splitter-size));
	position: relative;
}
.image-compare span {
	display: block;
	position: absolute;
	top: 0;
	left: calc(-1 * var(--expand));
	bottom: calc(-1 * var(--expand));
	width: calc(var(--expand) + 50% + var(--splitter-size) / 2);
	max-width: calc(var(--expand) + 100%);
	min-width: var(--handle-size);
	padding-left: var(--expand);
	padding-bottom: var(--expand);
	background: linear-gradient(135deg, transparent 0, transparent 50%, var(--splitter-color) 50%, var(--splitter-color)) 100% 100% / var(--handle-size) var(--handle-size) no-repeat;
	resize: horizontal;
	overflow: hidden;
}
.image-compare img {
	display: block;
	height: 100%;
	user-select: none;
}
.image-compare > img {
	max-width: 100%;
	height: auto;
}
.image-compare span::after {
	content: "";
	display: block;
	position: absolute;
	top: 0;
	right: 0;
	bottom: var(--expand);
	border-right: var(--splitter-size) solid var(--splitter-color);
}

/* Safari Workaround */

@supports (-webkit-hyphens: none) { /* Safari */
	.image-compare span {
		width: var(--handle-size);
	}
}

/* Accessibility Fallback #1 */

@keyframes pingpong {
	0%, 100% {
		width: calc(var(--expand) + 50% + var(--splitter-size) / 2);
	}
	75% {
		width: var(--handle-size);
	}
	25% {
		width: calc(var(--expand) + 100%);
	}
}
.image-compare:focus span {
	background-image: none;
	animation: pingpong 5s linear infinite;
	resize: none;
}

/* Accessibility Fallback #2 */

@keyframes fade {
	0%, 22%, 77%, 100% {
		opacity: 1;
	}
	27%, 73% {
		opacity: 0;
	}
}
@media (prefers-reduced-motion: reduce) {
	.image-compare:focus span {
		width: calc(var(--expand) + 100%);
		animation-name: fade;
	}
	.image-compare:focus span::after {
		display: none;
	}
}

/* unimportant stuff below */

*,
::before,
::after {
	box-sizing: border-box;
}
:focus {
	outline: 1px dashed hotpink;
	outline-offset: 1px;
}
html {
	font-family: sans-serif;
	line-height: 1.5;
	font-size: 100%;
}
body {
	max-width: 24rem;
	margin: 1rem auto 6rem;
}
figure {
	padding: 1rem;
	background-color: #ccc;
}
figure,
p {
  margin: 0 0 1rem;
}

/* what follows is a hack so we only need one image */

span img {
  filter: grayscale(1);
}

              
            
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JS

              
                
              
            
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