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Smarter custom properties with Houdini’s new API

Transitions and data protection in CSS

CSS custom properties, also known as CSS variables, let you define your own properties in CSS and use their values throughout your CSS. While incredibly useful today, they have shortcomings that can make them hard to work with: they can take any value so they may be accidentally overridden with something unexpected, they always inherit their values from their parent, and you can't transition them. With Houdini's CSS Properties and Values API Level 1, now available in Chrome 78, these shortcomings are transcended, making CSS custom properties incredibly powerful!

What Is Houdini?

Before talking about the new API, let's talk about Houdini quickly. The CSS-TAG Houdini Task Force, better known as CSS Houdini or simply Houdini, exists to "develop features that explain the 'magic' of styling and layout on the web". The collection of Houdini specifications are designed to open up the power of the browser's rendering engine, allowing both deeper insight into our styles and the ability to extend our rendering engine. With this, typed CSS values in JavaScript and polyfilling or inventing new CSS without a performance hit are finally possible. Houdini has the potential to superpower creativity on the web.

CSS Properties and Values API Level 1

The CSS Properties and Values API Level 1 (Houdini Props and Vals) allows us to give structure to our custom properties. This is the current situation when using custom properties:

.thing {
--my-color: green;

Because custom properties don't have types, they can be overridden in unexpected ways. For example, consider what happens if you define --my-color with a URL.

.thing {
--my-color: url('not-a-color');
color: var(--my-color);

Here, because --my-color isn't typed, it doesn't know that a URL isn't a valid color value! When we use it, it falls back to default values (black for color, transparent for background). With Houdini Props and Vals, custom properties can be registered so that the browser knows what it should be!

Now, the custom property --my-color is registered as a color! This tells the browser what kinds of values are allowed and how it can type and treat that property!

Anatomy of a registered property

Registering a property looks like this:

name: '--my-color',
syntax: '<color>',
inherits: false,
initialValue: 'black',

It supports the following options:

name: string

The name of the custom property.

syntax: string

How to parse the custom property. You can find a complete list of possible values in the CSS Values and Units specification. Defaults to *.

inherits: boolean

Whether it inherits its parent's value. Defaults to true.

initialValue: string

Initial value of the custom property.

Taking a closer look at syntax. There are a number of valid options ranging from numbers to colors to <custom-ident> types. These syntaxes can also be modified by using the following values

  • Appending + signifies that it accepts a space-separated list of values of that syntax. For example, <length>+ would be a space-separated list of lengths
  • Appending# signifies that it accepts a comma-separated list of values of that syntax. For example, <color># would be a comma-separated list of colors
  • Adding | between syntaxes or identifiers signifies that any of the provided options are valid. For example, <color># | <url> | magic would allow either a comma-separated list of colors, a URL, or the word magic.


There are two gotchas with Houdini Props and Vals. The first is that, once defined, there's no way to update an existing registered property, and trying to re-register a property will throw an error indicating that 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 including an invalid property after a valid one won't fall back to the valid one; an invalid property will, however, fall back to the registered property's default.

Animating custom properties

A registered custom property provides a fun bonus beyond type checking: the ability to animate it! A basic animation example looks like this:

name: '--stop-color',
syntax: '<color>',
inherits: false,
initialValue: 'blue',

button {
--stop-color: red;
transition: --stop-color: 1s;

button:hover {
--stop-color: green;

When you hover over the button, it'll animate from red to green! Without registering the property, it'll jump from one color to the other Because, without being registered, the browser doesn't know what to expect between one value and the next and therefore can't guarantee the ability to transition them. This example can be taken a step further, though, to animate CSS gradients! The following CSS can be written with the same registered property:

button {
--stop-color: red;
background: linear-gradient(var(--stop-color), black);
transition: --stop-color 1s;

button:hover {
--stop-color: green;

This will animate our custom property that's part of the linear-gradient, thus animating our linear gradient. Check out the Glitch below to see the full code in action and play around with it yourself.


Houdini is on its way to browsers, and with it, entirely new ways of working with and extending CSS. With the Paint API already shipped and now Custom Props and Vals, our creative toolbox is expanding, allowing us to define typed CSS properties and use them to create and animate new and exciting designs. There's more on the way, too, in the Houdini issue queue where you can give feedback and see what's next for Houdini. Houdini exists to develop features that explain the "magic" of styling and layout on the web, so get out there and put those magical features to good use.

Photo by Maik Jonietz on Unsplash

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