Custom property fallback values

Using the var() function, you can define multiple
fallback values when the given variable is not yet
defined

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

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

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