axis

The scrollbar axis value can be any one of the following:

block

The scrollbar on the block axis of the scroll container, which is the axis in the direction perpendicular to the flow of text within a line. For horizontal writing modes, such as standard English, this is the same as y, while for vertical writing modes, it is the same as x. This is the default value.

inline

The scrollbar on the inline axis of the scroll container, which is the axis in the direction parallel to the flow of text in a line. For horizontal writing modes, this is the same as x, while for vertical writing modes, this is the same as y.

y

The scrollbar on the vertical axis of the scroll container.

x

The scrollbar on the horizontal axis of the scroll container.

inset

The inset value can be one or two values, which can be either auto or a <length-percentage>. It specifies an inset (positive) or outset (negative) adjustment of the scrollport. The inset is used to determine whether the element is in view which determines the length of the animation timeline. In other words, the animation lasts as long as the element is in the inset-adjusted view.

start

Inward offset from beginning of the scrollport.

end

Inward offset from end of the scrollport.

An anonymous view progress timeline is set on an element with class subject using animation-timeline: view(). The result is that the subject element animates as it moves upwards through the document as it is scrolled.

HTML

The HTML for the example is shown below.

<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-container">
    <div class="subject animation"></div>
  </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>
<div class="overlay top">inset start 50%</div>
<div class="overlay bottom">inset end 10%</div>

CSS

The subject element and content elements are minimally styled and the text content is given some basic font settings:

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

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

p {
  font-size: 1.5rem;
  line-height: 1.8;
}

To aid the understanding of the result, extra elements subject-container, top, and bottom have been used. The subject-container shows the bounds of the animation. And semi-transparent top and bottom overlays mark inset offsetted scrollport.

.subject-container {
  border: 2px dashed black;
  width: 300px;
  margin: 0 auto;
}

.overlay {
  position: fixed;
  width: 100%;
  background-color: #f5deb3aa;
  display: flex;
  font-size: 1.2rem;
  font-weight: bold;
  color: red;
  justify-content: flex-end;
}

.top {
  top: 0;
  height: 244px;
  align-items: end;
}

.bottom {
  top: 432px;
  height: 48px;
}

In the following code, the <div> with the class of subject is also given a class of animation. The grow animation causes the subject element to grow or shrink. The animation-timeline: view(block 55% 10%) is set to declare that it will be animated as it progresses through the view progress timeline provided by its scrolling ancestor (in this case the document's root element).

While scrolling down, note how the inset value of 50% 10% causes the animation to start at 10% from the bottom and finish at 50% from the top. As animation moves forward along the timeline the subject grows. Conversely, when scrolling up the animation proceeds in the reverse direction, starting at 50% from the top, moving backward through the animation, and ending at 10% from the bottom. So, as the animation happens backwards the subject shrinks.

An important point to remember is that the animation lasts as long as the subject element is in the view which has been set and to be offset using 50% 10% inset values.

.animation {
  animation-timeline: view(block 50% 10%);

  animation-name: grow;
  animation-fill-mode: both;
  animation-duration: 1ms; /* Firefox requires this to apply the animation */
  animation-timing-function: linear;
}

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

  to {
    transform: scaleX(1);
  }
}

Result

Scroll to see the subject element being animated.

• CSS scroll-driven animations
• Using CSS animations
• animation-timeline