https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-camel-cased-attribute, https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-dashed-attribute and https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-_webkit_cased_attribute all define their "pseudo-attributes" like this:

    partial interface CSSStyleDeclaration {
      [CEReactions] attribute [TreatNullAs=EmptyString] CSSOMString _dashed_attribute;
    };

Blink currently uses anonymous property getters/setters to handle these (see bug 700338) with a different value for the [TreatNullAs] extended attribute:

    [CEReactions, CallWith=ScriptState, RaisesException] setter void (DOMString property, [TreatNullAs=NullString] DOMString propertyValue);

We should use [TreatNullAs=EmptyString] instead.