Chrome has shipped scroll-triggered animations, and is the first browser to do so. If you update to Chrome 146, you can view the demo below, where the background of a square fades in over the duration of 300ms, but only once the whole element is within the viewport.

This is a bit different to how scroll-driven animations work, so in this article I’ll compare them, and then show you how scroll-triggered animations work.

Scroll-triggered animations play for a fixed duration once a certain scroll threshold has been surpassed. (Think JavaScript’s Intersection Observer API but for CSS animations.)

This differs from scroll-driven animations, where animation progression is synchronized with scroll progression (animation-timeline: scroll()) or the degree of intersection (animation-timeline: view()), and thus has no duration.

The key part is timeline-trigger: view() instead of animation-timeline: view(), which waits for the element to be within the threshold instead of measuring how much it’s within it and doing something accordingly. However, let’s start with the actual @keyframes animation, which sets the background:

/* Define the animation */
@keyframes fade-bg-in {
  to {
    background: currentColor;
  }
}

It’s set on the .square over the duration of 300ms:

.square {
  /* Declare animation */
  animation: fade-bg-in 300ms;
}

By default, CSS animations trigger when the declaration is applied, but in the expanded snippet below, timeline-trigger overwrites that behavior. Now the animation triggers when the element comes into view(). The --trigger is simply a dashed ident that acts as an identifier for the trigger, whereas entry 100% exit 0% is a timeline range. A timeline range specifies the scroll zone in which the animation activates and is allowed to remain active.

In this case, the animation triggers when the bottom edge of the .square enters the (entry 100%) and untriggers (assuming that it’s still running) when the top edge exits the scrollport (exit 0%). For clarity, entry 0% would trigger the animation when the top edge enters. entry handles the element coming in from the bottom of the scrollport, whereas exit handles it leaving through the top. It’s a bit confusing, but it’s easier to understand if I don’t over-explain it.

.square {
  /* Declare animation */
  animation: fade-bg-in 300ms;

  /* Animation trigger conditions */
  timeline-trigger: --trigger view() entry 100% exit 0%;
}

For animation-trigger, we first specify which trigger we’re talking about, and then we declare some settings (e.g., play-forwards):

.square {
  /* Declare animation */
  animation: fade-bg-in 300ms;

  /* Animation trigger conditions */
  timeline-trigger: --trigger view() entry 100% exit 0%;

  /* Animation trigger settings */
  animation-trigger: --trigger play-forwards;
}

The play-forwards keyword triggers the animation whenever the square becomes completely visible, and since we haven’t declared a fill mode for the animation (using animation-fill-mode or as part of the animation shorthand), which means that the square won’t retain the background after, the animation is more of a flash.

So, we need to build upon this to achieve different results.

animation-fill-mode vs. <animation-action>

First, a recap of what the different fill mode values do for animation-fill-mode or as part of the animation shorthand:

• forwards: the styles are retained after the animation.
• backwards: the styles are applied before the animation.
• both: both behaviors are applied.

Now, let’s assume that the <animation-action> is play-forwards (like before) and the fill mode is forwards (both would be redundant because background isn’t even set to begin with):

.square {
  animation: fade-bg-in 300ms forwards;
  timeline-trigger: --trigger view() entry 100% exit 0%;
  animation-trigger: --trigger play-forwards;
}

This causes the styles to be retained, but, if the square partially or completely exits the viewport and then reenters it, the animation restarts, which can cause a flash depending on how the animation ends, which is what happens in this instance.

There are two different ways to solve this…

The “lock-in” method: Use play-once instead of play-forwards, which, when combined with forwards, results in the animation playing once, never to restart, and then retaining the styles afterward.

.square {
  /* Play once */
  animation-trigger: --trigger play-once;

  /* Retain the styles */
  animation: fade-bg-in 300ms forwards;

  timeline-trigger: --trigger view() entry 100% exit 0%;
}

The “back-and-forth” method: play-forwards play-backwards animates the element normally when fully visible and in reverse when no longer fully visible. There’s no flash because the element animates backward as smoothly as it animates forward. In addition, even though the direction of the animation can change, the fill mode can remain at forwards instead of being set to both.

Why?

play-forwards means “play the animation from 0% to 100%” whereas play-backwards means “play the animation from 100% to 0%.” Meanwhile, as I mentioned earlier, the forwards fill mode means “retain the styles when the animation completes’”— well, this is regardless of whether the final keyframe is 0% or 100%.

.square {
  /* Play forward and backward, as appropriate */
  animation-trigger: --trigger play-forwards play-backwards;

  /* Retain the styles either way */
  animation: fade-bg-in 300ms forwards;

  timeline-trigger: --trigger view() entry 100% exit 0%;
}

play-forwards, play-once, and play-backwards aren’t the only keywords for <animation-action>. Here’s a quick rundown:

<animation-action>	Effect
none	For disabling triggers conditionally, on entry but not exit (or vice-versa), or handling multiple triggers with one animation-trigger
play-forwards	Allows the animation to play forward
play-backwards	Allows the animation to play backward
play-once	Forward or backward (whichever comes first)
play	Plays in the last specified direction, or forward if neither has been specified
pause	Pauses the animation
reset	Pauses the animation and sets progress to 0
replay	Sets progress to 0 but doesn’t pause the animation

These <animation-action>s not only allow for a significant amount of control over animations while scrolling, but different combinations of actions, fill modes, timeline ranges, and the fact that we can bake exit animations into @keyframes rules means that there are often multiple ways to achieve an outcome.

While scroll-triggered animations being made up of animation actions, fill modes, timeline ranges, and maybe more, might seem overcomplicated, the fact that these mechanics are decoupled enable us to reuse logic while maintaining flexibility, reducing repetition and making the mechanics more design system-friendly.

Consider three squares this time, and for a bit of added complexity, we declare scale: 70% (animates to initial) and define two rotative animations.

<div id="squares">
  <div class="square rotate-left"></div>
  <div class="square"></div>
  <div class="square rotate-right"></div>
</div>

/* Define animations */
@keyframes intensify {
  to {
    scale: initial;
    background: currentColor;
  }
}

@keyframes rotate-left {
  to {
    rotate: -5deg;
  }
}

@keyframes rotate-right {
  to {
    rotate: 5deg;
  }
}

.square {
  /* Set starting value */
  scale: 70%;
}

After that it’s more of the same, and while it’s obviously a more complex example, being able to merge values into shorthand properties and decouple them into longhand properties, as well as the decoupled nature of the different mechanics, facilitates flexibility but also reusability (in this case, to stagger various animations using the same animation trigger settings):

.square {
  /* Set starting value */
  scale: 70%;

  /* Define animation name */
  --base-animation: intensify;

  /* Declare animation */
  animation: var(--base-animation) 300ms forwards;

  /* Define animation trigger settings */
  --animation-trigger: --trigger play-forwards play-backwards;

  /* Declare for intensify, then for one of either rotate animations */
  animation-trigger: var(--animation-trigger), var(--animation-trigger);

  /* Declare animation trigger conditions (without timeline ranges) */
  timeline-trigger: --trigger view();

  /* Declare active range end */
  timeline-trigger-active-range-end: normal;

  /* Append other animations */
  &.rotate-left {
    animation-name: var(--base-animation), rotate-left;
  }

  &.rotate-right {
    animation-name: var(--base-animation), rotate-right;
  }

  /* Stagger activation ranges */
  &:first-child {
    timeline-trigger-activation-range-start: entry 33.3333%;
  }

  &:nth-child(2) {
    timeline-trigger-activation-range-start: entry 66.6666%;
  }

  &:last-child {
    timeline-trigger-activation-range-start: entry 99.9999%;
  }
}

Here’s a cleaner, more robust version that uses sibling-count() and sibling-index() (which lack Firefox support) to stagger the animations:

In this version, instead of setting timeline-trigger-activation-range-start on each individual square, we simply target .square and calculate the entry values on the fly:

/* Maximum entry ÷ number of squares */
--stagger-interval: calc(100% / sibling-count());

/* Current square’s index × stagger interval */
--entry: calc(sibling-index() * var(--stagger-interval));

/* Declare animation trigger conditions */
timeline-trigger: --trigger view() entry var(--entry) exit 0%;

Making one element trigger other elements

In this case, we’ll shift the trigger and its ranges to the first square, and have the other squares follow according to a staggered animation delay. As you can see, all animations are triggered by animation-trigger once 50% of the first square has entered (entry 50%) the viewport (view()). animation-trigger is triggered by timeline-trigger because the dashed ident (the aptly named --trigger) links them:

/* Define animations */
@keyframes intensify {
  to {
    scale: initial;
    background: currentColor;
  }
}

@keyframes rotate-left {
  to {
    rotate: -5deg;
  }
}

@keyframes rotate-right {
  to {
    rotate: 5deg;
  }
}

.square {
  /* Set starting value */
  scale: 70%;

  /* Define animation name */
  --base-animation: intensify;

  /* Maximum delay ÷ number of squares */
  --stagger-interval: calc(300ms / sibling-count());

  /* Current square’s index × stagger interval */
  --animation-delay: calc(sibling-index() * var(--stagger-interval));

  /* Declare animation */
  animation: var(--base-animation) 300ms var(--animation-delay) forwards;

  /* Define animation trigger settings */
  --animation-trigger: --trigger play-forwards play-backwards;

  /* Declare for intensify, then for one of either rotate animations */
  animation-trigger: var(--animation-trigger), var(--animation-trigger);

  &:first-child {
    /* Declare animation trigger conditions */
    timeline-trigger: --trigger view() entry 50%;

    /* Declare active range end */
    timeline-trigger-active-range-end: normal;
  }

  /* Append other animations */
  &.rotate-left {
    animation-name: var(--base-animation), rotate-left;
  }

  &.rotate-right {
    animation-name: var(--base-animation), rotate-right;
  }
}

One downside is that when animation-trigger is in play-backwards mode, the animations don’t stagger. This is because, I think, when the animation is reversed, the delay is included in that. This seems like an oversight to me, especially as that isn’t the case with animation-direction: reverse, but I could be completely wrong on this.

Understanding timeline ranges

Timeline ranges are a big part of scroll-triggered animations, but they’re a separate mechanic. For scroll-driven animations, you’ll want animation-range and its longhand properties. With scroll-triggered animations, the syntax is fundamentally the same but uses different properties and two different ranges. The activation range determines the scroll zone in which the animation triggers, while the active range determines the zone in which it holds up (even if not in the activation range anymore).

Timeline ranges are a bit heavy. However, view() entry 100% exit 0% (when fully visible) and view() contain (the same but also if larger than the viewport) will suffice most of the time.

But if you’re keen to dive in, animation-range, although it’s for scroll-driven animations, is lighter and offers a novice-level understanding of timeline ranges. After that, I recommend reading the Animation Triggers spec to cover the many intricacies of timeline ranges within the context of these scroll-triggered animations.

Another ingredient of scroll-triggered animations that’s also its own thing is the view() function, but this one’s easier to summarize here. Basically, when it comes to scroll-triggered animations, view() is the viewport. So if you had a 5rem sticky header, view(y 0 5rem) would make the timeline range factor that in along the y-axis.

Final thoughts

Scroll-triggered animations can be tricky because they’re similar to scroll-driven animations, they leverage older CSS features (mainly animation) as well as mechanics from other newer features (dashed idents, view(), timeline ranges), in addition to the CSS properties that are specific to scroll-triggered animations. There’s a whole lot happening at once.

I’m not sure how I feel about them, to be honest. They’re definitely cool, fun, and useful, but they’re also complicated, and it’ll be a while before I really start to rave about them.