var() - CSS: Cascading Style Sheets
var( <custom-property-name> , <declaration-value>?
... values <custom-property-name> a custom property’s name represented by an identifier that starts with two dashes.

Adding Properties to XBL-defined Elements - Archive of obsolete content
the general syntax is as follows: <binding id="element-name"> <content> -- content goes here -- </content> <implementation> <field name="field-name-1"/> <field name="field-name-2"/> <field name="field-name-3"/> <property name="property-name-1"/> <property name="property-name-2"/> <property name="property-name-3"/> .

TransitionEvent() - Web APIs
propertyname optional is a domstring containing the value of the property-name css property associated with the transition.