Duotone image reveal on hover with CSS mask and blend modes

import styles from "./Demo.module.scss";

/**
 * No hooks, no handlers, no event listeners: the whole effect lives in
 * Demo.module.scss. This stays a component only so the Lab can mount every
 * experiment the same way.
 */
export default function DuotoneImageReveal() {
  return (
    <div className={styles.wrap}>
      <figure className={styles.figure} tabIndex={0}>
        <div className={styles.frame}>
          <img
            className={styles.base}
            src="/lab/duotone-source.jpg"
            alt="The Christmas Shop 24 storefront: a deep green hero with a gold script wordmark."
            width={1200}
            height={750}
            // No loading="lazy": the whole demo is already gated on
            // intersection by LabMount, and a lazy image inside a
            // content-visibility: auto subtree never gets asked for at all.
            decoding="async"
          />

          {/* The duotone layer. Same picture as the one underneath, so it
              carries no alt text and is hidden from assistive technology. */}
          <span className={styles.duotone} aria-hidden>
            <img
              src="/lab/duotone-source.jpg"
              alt=""
              width={1200}
              height={750}
              decoding="async"
            />
          </span>
        </div>

        <figcaption className={styles.caption}>
          Hover or focus to let the colour back in
        </figcaption>
      </figure>
    </div>
  );
}

/* Registering the custom property is what makes it interpolate. Without this
   the browser treats --duo-r as an opaque string and the reveal snaps instead
   of growing. Distinctive name: registration is global, not module-scoped. */
@property --duo-r {
  syntax: "<percentage>";
  inherits: true;
  initial-value: 0%;
}

.wrap {
  display: grid;
  place-items: center;
  padding: clamp(20px, 4vw, 36px);
  max-width: 100%;
  max-height: 100%;
}

.figure {
  --duo-r: 0%;
  display: flex;
  flex-direction: column;
  width: min(100%, 520px);
  min-height: 0;
  max-height: 100%;
  transition: --duo-r 0.55s ease;
  border-radius: var(--r-md);

  &:hover,
  &:focus-visible {
    --duo-r: 100%;
  }
}

/* min-height: 0 lets the frame shrink inside a height-capped card stage;
   without it a flex item refuses to go below its content's size. */
.frame {
  position: relative;
  flex: 1;
  min-height: 0;
  overflow: hidden;
  border-radius: var(--r-md);
  border: 1px solid var(--line);
  box-shadow: var(--shadow-1);
  line-height: 0;
}

.base {
  width: 100%;
  height: 100%;
  object-fit: cover;
  aspect-ratio: 1200 / 750;
}

/* The duotone. `isolation: isolate` keeps the blend inside this layer, so the
   gradient tints the grayscale copy and never touches the colour original
   sitting underneath it. */
.duotone {
  position: absolute;
  inset: 0;
  isolation: isolate;
  display: block;

  /* A hole that opens from the centre. Transparent out to --duo-r, opaque
     14% later: that gap is the soft edge, and it is the one value here worth
     tuning by eye. */
  mask-image: radial-gradient(
    circle at 50% 50%,
    transparent var(--duo-r),
    #000 calc(var(--duo-r) + 14%)
  );
  -webkit-mask-image: radial-gradient(
    circle at 50% 50%,
    transparent var(--duo-r),
    #000 calc(var(--duo-r) + 14%)
  );

  img {
    width: 100%;
    height: 100%;
    object-fit: cover;
    filter: grayscale(1) contrast(1.15);
  }
}

/* The tint. `color` blend keeps the grayscale luminance and takes only the
   hue and saturation from the gradient, which is exactly what a duotone is. */
.duotone::after {
  content: "";
  position: absolute;
  inset: 0;
  mix-blend-mode: color;
  background: linear-gradient(140deg, var(--blush), var(--lavender));
}

.caption {
  margin-top: 12px;
  font-family: var(--mono);
  font-size: 11px;
  letter-spacing: 0.04em;
  color: var(--silver-500);
  text-align: center;
}

@media (prefers-reduced-motion: reduce) {
  /* Cut rather than grow. Nothing is lost: both layers show the same photo. */
  .figure {
    transition: none;
  }
}