animation-range-start CSS property

/* Keyword or length percentage value */
animation-range-start: normal;
animation-range-start: 20%;
animation-range-start: 100px;

/* Named timeline range value */
animation-range-start: cover;
animation-range-start: contain;
animation-range-start: cover 20%;
animation-range-start: contain 100px;

/* Global values */
animation-range-start: inherit;
animation-range-start: initial;
animation-range-start: revert;
animation-range-start: revert-layer;
animation-range-start: unset;

<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="animatedElement"></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>

.animatedElement {
  background-color: deeppink;

  animation: appear 1ms linear;

  animation-timeline: scroll();
  animation-range-start: -25%;
}

@keyframes appear {
  from {
    background-color: rebeccapurple;
    opacity: 0;
    transform: scaleX(0);
  }

  to {
    background-color: darkturquoise;
    opacity: 0.75;
    transform: scaleX(0.75);
  }
}

.animatedElement {
  height: 200px;
  margin: 0 auto;
  background-color: deeppink;
}

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

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

h1 {
  font-size: 3rem;
}

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

@supports not (animation-range-start: normal) {
  body::before {
    content: "Your browser does not support the 'animation-range-start' property.";
    color: black;
    background-color: wheat;
    display: block;
    text-align: center;
    padding: 1rem 0;
  }
}