CSS Custom Property

A CSS custom property is a variable declared in a style rule and read with var(). Unlike a preprocessor variable, it is a live part of the cascade: it inherits, it can be redefined per element, and it can be changed at runtime.

The behaviour that most often surprises people is where substitution happens. A var() reference is resolved at the element where the declaration using it lives, not where the variable was defined. So a rule at the document root that says --brand-color: var(--scoped-thing) resolves at the root — where the scoped value does not exist — and inherits an invalid value everywhere, no matter how many descendants define --scoped-thing. To make a token overridable inside a scope, the reference has to appear in the rule that ultimately applies to the element. See Scoped Styling.

Two further practical notes. A var() that resolves to nothing fails quietly: the property becomes invalid at computed-value time and falls back to inherited or initial, so a radius silently becomes zero and a colour silently becomes black. That is a Silent Failure a pixel comparison barely registers. And custom properties are strings until used, so they can hold fragments of any declaration — useful, and a way to build things no type system will check.