CSS variables that provoke invalid CSS declarations (for example --foo: ;) are set to unset.

Space toggle for numerical value: calc(1.5 var(--toggle-value, - .4))
with toggle value:

:root {
  --ON: initial
  --OFF: ;
}

Use variables for pure data, not CSS values as CSS values are often bound with a unit.

Relative values inherit as syntax tokens unless the property is registered. Registering a property as allows us to pass down ancestor font sizes.