In CSS, you can declare a custom property using two dashes as a prefix for the property name, or by using the @property at-rule. The following sections describe how to use these two methods.

A custom property prefixed with two dashes begins with --, followed by the property name (e.g., --my-property), and a property value that can be any valid CSS value. Like any other property, this is written inside a ruleset. The following example shows how to create a custom property --main-bg-color and uses a <named-color> value of brown:

section {
  --main-bg-color: brown;
}

The selector given to the ruleset (<section> elements in the example above) defines the scope in which the custom property can be used. For this reason, a common practice is to define custom properties on the :root pseudo-class, so that it can be referenced globally:

:root {
  --main-bg-color: brown;
}

This doesn't always have to be the case: you maybe have a good reason for limiting the scope of your custom properties.

The @property at-rule allows you to be more expressive with the definition of a custom property with the ability to associate a type with the property, set default values, and control inheritance. The following example creates a custom property called --logo-color which expects a <color>:

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

If you want to define or work with custom properties in JavaScript instead of directly in CSS, there is a corresponding API for this purpose. You can read about how this works in the var() function in place of a standard property value:

details {
  background-color: var(--main-bg-color);
}

Let's start with some HTML that we would like to apply some styles to. There is a <div> that acts as a container that includes some child elements, some with nested elements:

<div class="container">
  <div class="one">
    <p>One</p>
  </div>
  <div class="two">
    <p>Two</p>
    <div class="three">
      <p>Three</p>
    </div>
  </div>
  <input class="four" placeholder="Four" />
  <textarea class="five">Five</textarea>
</div>

We will use the following CSS to style a few different elements based on their classes (some layout rules are not shown below so we can focus on colors). Depending on their classes, we're giving elements cornflowerblue or aquamarine background colors:

/* Set fonts, borders and padding */
body {
  font-family: sans-serif;
  color: white;
}

div,
input,
textarea {
  border: 2px black solid;
  padding: 4px;
  margin: 4px;
}

.container {
  display: grid;
  gap: 10px;
}

/* For each class, set some colors */
.one {
  background-color: cornflowerblue;
}
.two {
  color: black;
  background-color: aquamarine;
}
.three {
  background-color: cornflowerblue;
}
.four {
  background-color: cornflowerblue;
}
.five {
  background-color: cornflowerblue;
}

This produces the following result:

There's an opportunity to use custom properties to replace repetitive values across these rules. After defining --main-bg-color in the .container scope and referencing its value in multiple places, the updated styles look like this:

/* Define --main-bg-color here */
.container {
  --main-bg-color: cornflowerblue;
}

/* For each class, set some colors */
.one {
  background-color: var(--main-bg-color);
}
.two {
  color: black;
  background-color: aquamarine;
}
.three {
  background-color: var(--main-bg-color);
}
.four {
  background-color: var(--main-bg-color);
}
.five {
  background-color: var(--main-bg-color);
}

For some CSS declarations, it is possible to declare this higher in the cascade and let CSS inheritance solve this problem. For non-trivial projects, this is not always possible. By declaring a custom property on the :root pseudo-class and using it where needed throughout the document, a CSS author can reduce the need for repetition:

/* Define --main-bg-color here */
:root {
  --main-bg-color: cornflowerblue;
}

/* For each class, set some colors */
.one {
  background-color: var(--main-bg-color);
}
.two {
  color: black;
  background-color: aquamarine;
}
.three {
  background-color: var(--main-bg-color);
}
.four {
  background-color: var(--main-bg-color);
}
.five {
  background-color: var(--main-bg-color);
}

This leads to the same result as the previous example, yet allows for one canonical declaration of the desired property value (--main-bg-color: cornflowerblue;), which is very useful if you want to change the value across the entire project later.

A custom property defined using two dashes -- instead of @property always inherits the value of its parent. This is demonstrated in the following example:

<div class="one">
  <p>One</p>
  <div class="two">
    <p>Two</p>
    <div class="three"><p>Three</p></div>
    <div class="four"><p>Four</p></div>
  </div>
</div>

div {
  color: black;
  font-family: sans-serif;
  width: 75%;
  height: 80%;
  margin: 4px;
  border: 2px black solid;
  display: inline-block;
}

p {
  margin: 0;
}

.one {
  height: 250px;
}

.two {
  height: 80%;
}

.three {
  height: 40%;
}

.four {
  height: 40%;
}

div {
  background-color: var(--box-color);
}

.two {
  --box-color: cornflowerblue;
}

.three {
  --box-color: aquamarine;
}

The results of var(--box-color) depending on inheritance are as follows:

• class="one": invalid value, which is the default value of a custom property defined in this way
• class="two": cornflowerblue
• class="three": aquamarine
• class="four": cornflowerblue (inherited from its parent)

One aspect of custom properties that the examples above demonstrate is that they don't behave exactly like variables in other programming languages. The value is computed where it is needed, not stored and reused in other places of a stylesheet. For instance, you cannot set a property's value and expect to retrieve the value in a sibling's descendant's rule. The property is only set for the matching selector and its descendants.

The @property at-rule lets you explicitly state whether the property inherits or not. The following example creates a custom property using the @property at-rule. Inheritance is disabled, there's a <color> data type defined, and an initial value of cornflowerblue.

The parent element sets --box-color to a value of green and uses --box-color as a value for its background color. The child element also uses background-color: var(--box-color), and we would expect it to have the color green if inheritance was enabled (or if it was defined using the double dash syntax).

<div class="parent">
  <p>Parent element</p>
  <div class="child">
    <p>Child element with inheritance disabled for --box-color.</p>
  </div>
</div>

div {
  color: white;
  font-family: sans-serif;
  width: 200px;
  height: 200px;
  margin: 4px;
  padding: 8px;
  border: 2px black solid;
  display: inline-block;
}

@property --box-color {
  syntax: "<color>";
  inherits: false;
  initial-value: cornflowerblue;
}

.parent {
  --box-color: green;
  background-color: var(--box-color);
}

.child {
  width: 80%;
  height: 40%;
  background-color: var(--box-color);
}

Because inherits: false; is set in the at-rule, and a value for the --box-color property is not declared within the .child scope, the initial value of cornflowerblue is used instead of green that would have been inherited from the parent:

You can define fallback values for custom properties using the var() function, and the initial-value of the @property at-rule.

Using the Shadow DOM.

The first argument to the function is the name of the custom property. The second argument to the function is an optional fallback value, which is used as the substitution value when the referenced custom property is invalid. The function accepts two parameters, assigning everything following the first comma as the second parameter. If the second parameter is invalid, the fallback will fail. For example:

.one {
  /* Red if --my-var is not defined */
  color: var(--my-var, red);
}

.two {
  /* pink if --my-var and --my-background are not defined */
  color: var(--my-var, var(--my-background, pink));
}

.three {
  /* Invalid: "--my-background, pink" */
  color: var(--my-var, --my-background, pink);
}

Including a custom property as a fallback, as seen in the second example above (var(--my-var, var(--my-background, pink))), is the correct way to provide more than one fallback with var(). You should be aware of the performance impact of this method, however, as it takes more time to parse through the nested variables.

Aside from using var(), the initial-value defined in the @property at-rule can be used as a fallback mechanism. In fact, we've already seen this in the @property inheritance section.

The following example sets an initial value of --box-color to cornflowerblue using the @property at-rule. In the ruleset following the at-rule, we want to set --box-color to aquamarine, but there's a typo in the value name. The same is true for the third <div> where we've used 2rem for the custom property that's expecting a valid <color> value. Both 2rem and aqumarine are invalid color values, so the initial value of cornflowerblue is applied:

@property --box-color {
  syntax: "<color>";
  initial-value: cornflowerblue;
  inherits: false;
}

.one {
  --box-color: aquamarine;
  background-color: var(--box-color);
}

.two {
  --box-color: aqumarine;
  background-color: var(--box-color);
}

.three {
  --box-color: 2rem;
  background-color: var(--box-color);
}

div {
  color: white;
  font-family: sans-serif;
  width: 100px;
  height: 100px;
  margin: 4px;
  padding: 8px;
  border: 2px black solid;
  display: inline-block;
}
.one {
  color: black;
}

<div class="one">
  <p>One</p>
</div>
<div class="two">
  <p>Two.</p>
</div>
<div class="three">
  <p>Three.</p>
</div>

However, when the values of custom properties are parsed, the browser doesn't yet know where they will be used, so it must consider nearly all values as valid. Unfortunately, these valid values can be used, via the var() functional notation, in a context where they might not make sense. Properties and custom variables can lead to invalid CSS statements, leading to the concept of valid at computed time.

When the browser encounters an invalid var() substitution, then the inherited value of the property is used. This example is just like the last one, except we use a custom property.

The browser substitutes the value of --text-color in place of var(--text-color), but 16px is not a valid property value for color. After substitution, the property doesn't make sense., so the browser handles this situation in two steps:

1. Check if the property color is inheritable. It is, but this <p> doesn't have any parent with the color property set. So we move on to the next step.
2. Set the value to its default initial value, which is black.

<p>This paragraph is initially black.</p>

:root {
  --text-color: 16px;
}

p {
  color: blue;
}

p {
  color: var(--text-color);
}

For such cases, the @property at-rule can prevent unexpected results by allowing to define the initial value of the property:

<p>This paragraph is initially black.</p>

@property --text-color {
  syntax: "<color>";
  inherits: false;
  initial-value: cornflowerblue;
}

:root {
  --text-color: 16px;
}

p {
  color: blue;
}

p {
  color: var(--text-color);
}

To use the values of custom properties in JavaScript, it is just like standard properties.

/ get variable from inline style
element.style.getPropertyValue("--my-var");

/ get variable from wherever
getComputedStyle(element).getPropertyValue("--my-var");

/ set variable on inline style
element.style.setProperty("--my-var", jsVar + 4);

• Custom property syntax
• @property at-rule
• var()
• CSS Properties and Values API
• CSS custom properties for cascading variables module