The Tool Desk
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Declare a custom property and use it with var()
A custom property name begins with two hyphens. Put its declaration inside a CSS rule, then reference it within another property’s value:
:root {
--brand-color: rebeccapurple;
--space-unit: 0.5rem;
}
.button {
background-color: var(--brand-color);
padding: calc(var(--space-unit) * 2);
}Here, --brand-color and --space-unit are custom properties. The var() function substitutes their values into the declarations that use them. You can use a custom property in calculations such as calc(), provided the resulting value is valid for the consuming property.
Custom property names are case-sensitive: --brand-color and --Brand-color are different names. MDN describes double-hyphen custom properties as participating in the cascade and inheriting from their parent: MDN: Using CSS custom properties (variables).
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Put shared tokens on :root—or scope them locally
:root is a common place for site-wide tokens because its custom properties can inherit into the document’s descendants. It is a convention, not a requirement. Declare a property on a component when the value should be local to that component and its descendants.
.card {
--surface-color: white;
background-color: var(--surface-color);
}
.card--dark {
--surface-color: #222;
}A descendant of an element with --surface-color ordinarily inherits its value. A declaration on a more specific element in the relevant ancestry can provide another value according to the normal cascade. The token belongs to the element where it is declared and applies through inheritance; it is not a global text replacement, and a sibling does not inherit a value declared only on another sibling.
Use fallbacks when a token may be unavailable
The optional second argument to var() is used when the referenced custom property has the guaranteed-invalid value, for example when an ordinary, unregistered custom property has not been set:
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.notice {
color: var(--notice-color, #333);
}Fallbacks can be nested when one token should fall back to another:
.panel {
background-color: var(--panel-color, var(--surface-color, white));
}This is CSS fallback behavior at runtime, not a polyfill. A browser that does not support custom properties will not gain support for var() from adding a fallback. MDN says var() has been available across browsers since April 2017; check compatibility data for the browser versions and embedded webviews you need to support: MDN: var().
Know what happens when a substituted value is invalid
A custom property can hold a token sequence that does not make sense for the property where it is used. CSS checks the consuming declaration after substitution. For example, if --text-color is set to 16px, then color: var(--text-color) becomes invalid at computed-value time. The var() fallback does not repair a value that exists but is invalid for color.
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Use tokens consistently and keep their intended value types clear in their names and usage. A color token should hold a color; a spacing token should hold a length or another value appropriate for its consumer.
Use @property when a token needs explicit rules
Ordinary custom properties are the simple default for reusable design tokens. The optional @property registration feature lets you declare a custom property’s syntax, inheritance behavior, and initial value:
@property --progress {
syntax: "<percentage>";
inherits: false;
initial-value: 0%;
}
.progress-bar {
width: var(--progress);
}Registration makes the value’s intended type and inheritance explicit. Registered typed values can also be animated. Because a registered property can have an initial value, an unset property with a non-universal syntax may use that initial value rather than the var() fallback. Choose registration when these constraints or animation behavior are useful; it is not necessary for ordinary tokens.
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MDN marks @property Baseline 2024. That is documentation guidance, not a guarantee for every older browser or embedded webview, so check the compatibility tables for your target audience: MDN: @property and MDN: CSS Properties and Values API.
Understand where custom properties can be used
var() substitutes values within a CSS property value. It cannot parameterize selectors, property names, media-query conditions, or container-query conditions. For example, define responsive breakpoints directly in the query rather than attempting to store a breakpoint in a custom property:
@media (min-width: 48rem) {
.layout {
gap: var(--space-unit);
}
}The custom property can be used in the declaration inside the query; it cannot supply the query’s 48rem condition.
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