CSSStyleDeclaration: Setting only a value or a priority

The current specification of CSSStyleDeclaration.setProperty makes it necessary to call either 
'getPropertyPriority' or 'getPropertyValue' in case only a property value or a property priority is to be modified.
For the purpose of faster and simpler low level style transformations, I would like to see either: 
1) the CSSStyleDeclaration interface to be extended with the functions 'setPropertyValue' and 'setPropertyPriority', or
2)
 the CSSStyleDeclaration.setProperty function to accept 'null' for the 
property value or property priority as an indication that the 
value/priority should not be changed.

PJS, Firefox add-on developer.