Interpolating Numeric CSS Variables

We can make variables in CSS pretty easily:

:root {
  --scale: 1;
}

And we can declare them on any element:

.thing {
  transform: scale(--scale);
}

Even better for an example like this is applying the variable on a user interaction, say :hover:

:root {
  --scale: 1;
}

.thing {
  height: 100px;
  transform: scale(--scale);
  width: 100px;
}

.thing:hover {
  --scale: 3;
}

But if we wanted to use that variable in an animation… nada.

:root {
  --scale: 1;
}

@keyframes scale {
  from { --scale: 0; }
  to { --scale: 3; }
}

/* Nope! */
.thing {
  animation: scale .25s ease-in;
  height: 100px;
  width: 100px;
}

That’s because the variable is recognized as a string and what we need is a number that can be interpolated between two numeric values. That’s where we can call on @property to not only register the variable as a custom property, but define its syntax as a number:

@property --scale {
  syntax: "<number>";
  initial-value: 1;
  inherits: true;
}

Now we get the animation!

You’re going to want to check browser support since @property has only landed in Chrome (starting in version 85) as of this writing. If you’re going to use it, probably best to use it where it’s supported. Looks like we can use the properties from @property to sniff that out via @supports:

@supports (inherits: true) {
  /* Styles for supporting browsers. */
}

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Interpolating Numeric CSS Variables originally published on CSS-Tricks, which is part of the DigitalOcean family. You should get the newsletter.

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