[css-variables] CSSVariablesMap enumeration

Hello,

I've taken over the CSS Variables implementation in Blink. I have some
concerns/questions regarding iterating over a CSSVariablesMap object.

At the moment enumeration over a CSSVariablesMap is only indicated in an
example.

var customProps = el.style.var;
for(customPropName in customProps) {
  if( knownCustomPropName(customPropName) ) {
    var customPropValue = customProps[customPropName];
    /* Processing code here. */
  }
}

It is hard to find a concrete specification that it has this behaviour.
Previously enumeration was implied by the presence of a getter interface
method. Is enumeration inherited as part of the MapClass interface?


I'm concerned this definition of CSSVariablesMap where "get" and "set" are
used in place of the usual getter and setter interface is introducing
strange behaviours into the spec when combined with enumeration. Example:

for (var i in el.style.var) {
  // e.style.var[i] is undefined.
}

Tab mentioned previously that getters and setters were discarded with the
example that you could add a property to the CSSVariablesMap prototype and
it would appear as a variable when it is not.
Example:
CSSVariablesMap.prototype.foo = "bar";
// el.style.var.foo returns "bar".

I'm curious whether the reasoning to accommodate this is to allow "monkey
patching" of the CSSVariablesMap interface. One can still introduce "fake"
variables by overriding the existing prototype methods. Personally I'm not
sure it's worth the effect it is having on the CSSOM API.


Link to draft spec: http://dev.w3.org/csswg/css-variables

Thanks