How Can I Animate Multiple Instances of a CSS Custom Property Using a Single Animation?

Animating Custom Properties in CSS

In CSS, custom properties, also known as CSS variables, offer a robust mechanism for defining and reusing styling values throughout a project. They provide the ability to dynamically change a style property based on a predefined value or variable. However, when it comes to animations, custom properties can sometimes pose challenges.

One such challenge arises when attempting to animate multiple instances of a particular property using a single animation. By leveraging custom properties, developers aim to have all subsequent instances affected by the same animation. However, in certain cases, only one instance undergoes the desired animation, while the remaining instances remain unaffected.

Understanding Custom Properties in Animations

The issue stems from the fundamental behavior of custom properties in animations. When defined using the traditional CSS syntax, custom properties are treated as static values during the animation. This means that the property values are not interpolated over the animation duration, resulting in a sudden change rather than a gradual transition.

Solution: Using the @property Rule

To overcome this limitation, developers can employ the @property rule in conjunction with custom properties. The @property rule allows for defining variables by specifying their types and providing initial values. By declaring a custom property as a number, for example, the browser can interpret the property as a numerical value and enable smooth transitions and animations.

Example Implementation:

Consider the following example, where we aim to create a blinking div element using a custom property for opacity:

@property --opacity {
  syntax: '<number>'; /* Define as type number for transition */
  initial-value: 0;
  inherits: false;
}

@keyframes fadeIn {
  50% {--opacity: 1}
}

#test {
  width: 100px;
  height: 200px;
  background-color: black;
  animation: fadeIn 1s infinite;
}

#test div {
  width: 20px;
  height: 20px;
  margin: auto;
  background-color: white;
  opacity: var(--opacity);
}

Using this approach, the custom property --opacity is defined as a number, allowing the animation to interpolate the opacity value smoothly over the duration of the animation, achieving the desired blinking effect.

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