scroller

The value for indicating the scroller element that will provide the scroll progress timeline can be any one of the following:

nearest

The nearest ancestor of the current element that has scrollbars on either axis. This is the default value.

root

The root element of the document.

self

The current element itself.

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.

<scroll()> = 
  scroll( [ <scroller> || <axis> ]? )  

<scroller> = 
  root     |
  nearest  

<axis> = 
  block       |
  inline      |
  vertical    |
  horizontal  

In this example, the #square element is animated using an anonymous scroll progress timeline, which is applied to the element to be animated using the scroll() function. The timeline in this particular example is provided by the nearest parent element that has (any) scrollbar, from the scrollbar in the block direction.

HTML

The HTML for the example is shown below.

<div id="container">
  <div id="square"></div>
  <div id="stretcher"></div>
</div>

CSS

The CSS below defines a square that rotates in alternate directions according to the timeline provided by the animation-timeline property. In this case, the timeline is provided by scroll(block nearest), which means that it will select the scrollbar in the block direction of the nearest ancestor element that has scrollbars; in this case the vertical scrollbar of the "container" element.

#square {
  background-color: deeppink;
  width: 100px;
  height: 100px;
  margin-top: 100px;
  position: absolute;
  bottom: 0;

  animation-name: rotateAnimation;
  animation-duration: 1ms; /* Firefox requires this to apply the animation */
  animation-direction: alternate;
  animation-timeline: scroll(block nearest);
}

@keyframes rotateAnimation {
  from {
    transform: rotate(0deg);
  }
  to {
    transform: rotate(360deg);
  }
}

The CSS for the container sets its height to 300px and we also set the container to create a vertical scrollbar if it overflows. The "stretcher" CSS sets the block height to 600px, which forces the container element to overflow. These two together ensure that the container has a vertical scrollbar, which allows it to be used as the source of the anonymous scroll progress timeline.

#container {
  height: 300px;
  overflow-y: scroll;
  position: relative;
}

#stretcher {
  height: 600px;
}

Result

Scroll to see the square element being animated.

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