animation-range

/* single keyword or length percentage value */
animation-range: normal; /* Equivalent to normal normal */
animation-range: 20%; /* Equivalent to 20% normal */
animation-range: 100px; /* Equivalent to 100px normal */

/* single named timeline range value */
animation-range: cover; /* Equivalent to cover 0% cover 100% */
animation-range: contain; /* Equivalent to contain 0% contain 100% */
animation-range: cover 20%; /* Equivalent to cover 20% cover 100% */
animation-range: contain 100px; /* Equivalent to contain 100px cover 100% */

/* two values for range start and end */
animation-range: normal 25%;
animation-range: 25% normal;
animation-range: 25% 50%;
animation-range: entry exit; /* Equivalent to entry 0% exit 100% */
animation-range: cover cover 200px; /* Equivalent to cover 0% cover 200px */
animation-range: entry 10% exit; /* Equivalent to entry 10% exit 100% */
animation-range: 10% exit 90%;
animation-range: entry 10% 90%;

<div class="content">
  <h1>Content</h1>

  <p>
    Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod
    tempor incididunt ut labore et dolore magna aliqua. Risus quis varius quam
    quisque id. Et ligula ullamcorper malesuada proin libero nunc consequat
    interdum varius. Elit ullamcorper dignissim cras tincidunt lobortis feugiat
    vivamus at augue.
  </p>

  <p>
    Dolor sed viverra ipsum nunc aliquet. Sed sed risus pretium quam vulputate
    dignissim. Tortor aliquam nulla facilisi cras. A erat nam at lectus urna
    duis convallis convallis. Nibh ipsum consequat nisl vel pretium lectus.
    Sagittis aliquam malesuada bibendum arcu vitae elementum. Malesuada bibendum
    arcu vitae elementum curabitur vitae nunc sed velit.
  </p>

  <div class="subject animation"></div>

  <p>
    Adipiscing enim eu turpis egestas pretium aenean pharetra magna ac. Arcu
    cursus vitae congue mauris rhoncus aenean vel. Sit amet cursus sit amet
    dictum. Augue neque gravida in fermentum et. Gravida rutrum quisque non
    tellus orci ac auctor augue mauris. Risus quis varius quam quisque id diam
    vel quam elementum. Nibh praesent tristique magna sit amet purus gravida
    quis. Duis ultricies lacus sed turpis tincidunt id aliquet. In egestas erat
    imperdiet sed euismod nisi. Eget egestas purus viverra accumsan in nisl nisi
    scelerisque. Netus et malesuada fames ac.
  </p>
</div>

.subject {
  width: 300px;
  height: 200px;
  margin: 0 auto;
  background-color: deeppink;
}

.content {
  width: 75%;
  max-width: 800px;
  margin: 0 auto;
}

p,
h1 {
  font-family: Arial, Helvetica, sans-serif;
}

h1 {
  font-size: 3rem;
}

p {
  font-size: 1.5rem;
  line-height: 1.5;
}

.animation {
  view-timeline: --subjectReveal block;
  animation-timeline: --subjectReveal;

  animation-name: appear;
  animation-range: entry 10% contain 25%;
  animation-fill-mode: both;
  animation-duration: 1ms; /* Firefox requires this to apply the animation */
}

@keyframes appear {
  from {
    opacity: 0;
    transform: scaleX(0);
  }

  to {
    opacity: 1,
    transform: scaleX(1);
  }
}