Browsers totally ignore CSS property/value declarations that they don't understand – falling back to earlier declarations. But sometimes variables look valid at parse-time, and are later discovered to be invalid at computed value time.

• Older browsers (without CSS Variable support) will always ignore declarations that contain variables.
• Modern browsers (with CSS Variable support) will ignore if the syntax is clearly invalid at parse-time. Otherwise:
  1. [Undefined] If the value is missing (or set to initial), it will try the provided fallback.
  2. [Type Error] If the final value doesn't match the property, it becomes unset — falling back to the inherited or default value.