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My New Favorite CSS Trick: will-change

Thea on March 13, 2024

I recently discovered a handy CSS trick that's made a noticeable difference in how I handle animations and transitions. Meet the useful will-change...

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Ben Halpern • Mar 13

Big +1. I've used this (again judiciously) alongside requestAnimationFrame in my JavaScript to massively improve performance on an app which needed it.

Thea • Mar 13

Yes, I agree! Using will-change alongside requestAnimationFrame is such an amazing combo for silky smooth animations. Glad to hear it massively improved the performance of your app!:)

Ben Halpern • Mar 13

Yeah — specifically the performance improvements were needed on mobile.

Phones have come a long way but are still sometimes resource-constrained.

Bayu Angora • Mar 13

I use this code for darkmode purpose:

* {
color: #ddd;
transition: color 1s, background 1s, border 1s, text-shadow 1s;
}
body {
display: flex;
flex-direction: column;
justify-content: center;
background: #000;
}

Is it good if I add will-change to that code like this?

* {
color: #ddd;
transition: color 1s, background 1s, border 1s, text-shadow 1s;
will-change: transition, color, background, border, text-shadow;
}
body {
display: flex;
flex-direction: column;
justify-content: center;
background: #000;
will-change: background;
}

Thea • Mar 14

Applying will-change to the * selector is generally not recommended, as it can lead to unneeded optimizations across too many elements. Instead, try using will-change only on the specific elements that will animate or change colors/backgrounds/etc during the dark mode transitions.

Matthijs Wensveen • Mar 20

Why can't the browser figure this out itself when parsing the css? It literally says "when you hover element X, change properties Y and Z". I wonder if most browsers don't already do this. It can't hurt to be explicit, of course, and maybe there are cases that are hard to detect, but it feels like this shouldn't be necessary most of the times.

I had never heard of this property before, so I learned something today. Thanks!

Mardeg • Mar 15

Will this be useful for SMIL animated inline SVG elements?
If I have a circle element that will move vertically and change transparency should I add will-change: cy, fill-opacity; to the CSS for it?

Thea • Mar 15

Yes, will-change can be very useful for optimizing animated SVG elements. For your example of a circle moving vertically with changing opacity, adding will-change: cy, fill-opacity; to that circle is an appropriate use case.

Arnaud Dagnelies • Mar 13 • Edited

I never used that. I wonder how much difference it makes in practice. Also, if I understand it right after digging into it a bit, applying it on too many elements may hurt performance. Since it's difficult to estimate the benefit / cost, it's quite tricky to use.

Thea • Mar 13 • Edited

You're right - will-change needs to be used carefully. Applying it carelessly can hurt performance more than help.

The key is using it only on elements with complex animations. Using it too much or leaving it on when not needed can slow things down.

In practice, will-change can greatly improve animation performance for elements that move, change transparency, or reposition a lot. You'll notice the biggest improvements on less powerful devices or when many animated elements are on the screen at once.

I'd recommend using will-change only on the specific elements being animated. That way, you get the performance benefits while avoiding potential slowdowns.
Thanks for bringing up this point!

Elly Okello • Mar 20

congratulations for an amazing piece ,learned about a new property ,will-change .why its important to give browser heads up of when intending to change certain properties. I'll sure try it out

João Angelo • Mar 20

Hi Thea,
Your tips are very useful
Thanks for sharing

Jeff Chavez • Mar 16

Thanks for making us aware and the caution given.