Re: [css-variables] do we need to have the CSSVariableMap at all?

An alternative to CSSVariableMap, if we find that we don't need the 
ability to expose a Map-like object representing the custom properties 
on a declaration, but we do want to save authors the hassle of 
prepending "var-" to their variable names to construct the custom 
property names, we could add

   DOMString getVariableValue(DOMString variableName);
   void setVariable(DOMString variableName, DOMString value,
                    optional DOMString priority);

to CSSStyleDeclaration.