Fallback

A backup value used when a token cannot be resolved in the current context.

In CSS, the var() function accepts a second argument as a fallback value:

button {
    background-color: var(--action-primary-bgColor, #0000FF);
}

In this example, if --action-primary-bgColor is not defined within the current scope, the browser uses #0000FF instead. Fallbacks can also reference another custom property:

button {
    background-color: var(--action-primary-bgColor, var(--color-blue-500));
}