Using the CSS properties and values API

Registering a custom property allows you to tell the browser how the custom property should behave; what types are allowed, whether the custom property inherits its value, and what the default value of the custom property is. There are two ways to register a property, in JavaScript or in CSS.

The following will register a CSS custom property named --my-prop using CSS.registerProperty. --my-prop will use the CSS color syntax, it will have a default value of #c0ffee, and it will not inherit its value:

window.CSS.registerProperty({
  name: "--my-prop",
  syntax: "<color>",
  inherits: false,
  initialValue: "#c0ffee",
});

The same registration can take place in CSS. The following will register a CSS custom property named --my-prop using the @property at-rule. --my-prop will use the CSS color syntax, it will have a default value of #c0ffee, and it will not inherit its value:

@property --my-prop {
  syntax: "<color>";
  inherits: false;
  initial-value: #c0ffee;
}

One of the advantages of registering a property is that the browser now knows how to handle your custom property through things like transitions! When a property isn't registered, the browser doesn't know how to treat it, so it assumes that any value can be used and therefore can't animate it. When a property has a registered syntax, though, the browser can optimize around that syntax, including being able to animate it!

In this example, the custom property --registered has been registered using the syntax <color> and then used in a linear gradient. That property is then transitioned on hover or focus to a different color. Notice that the transition works with the registered property but not the unregistered one!

While not functionally accurate, a good way to think about the difference between the unregistered property in the above example and the registered property is the difference between a <custom-ident> and a number when trying to animate height. You cannot transition or animate from auto to a number because the browser doesn't know the value of auto until it's calculated. With an unregistered property, the browser likewise doesn't know what the value may be until it's calculated, and because of that, it can't set up a transition from one value to another. When registered, though, you've told the browser what type of value it should expect, and because it knows that, it can then set up the transitions properly.

There are two gotchas when registering a property. The first is that, once a property is registered, there's no way to update it, and trying to re-register it with JavaScript will throw an error indicating it's already been defined.

Second, unlike standard properties, registered properties aren't validated when they're parsed. Rather, they're validated when they're computed. That means both that invalid values won't appear as invalid when inspecting the element's properties, and that including an invalid property after a valid one won't fall back to the valid property. An invalid property will, however, fall back to its registered default.