https://developer.mozilla.org/en-US/docs/Web/API/CSSVariableReferenceValue/fallback
CSSVariableReferenceValue: fallback property - Web APIs | MDN
The fallback read-only property of the CSSVariableReferenceValue interface returns the custom property fallback value of the CSSVariableReferenceValue.
web apiscssvariablereferencevaluefallbackpropertymdn
https://developer.mozilla.org/en-US/docs/Web/API/CSSVariableReferenceValue/CSSVariableReferenceValue
CSSVariableReferenceValue: CSSVariableReferenceValue() constructor - Web APIs | MDN
Creates a new CSSVariableReferenceValue.
constructor webcssvariablereferencevalueapismdn