Quote from issue 580323  #6 ;
> Per the CSSOM spec, we should be generating actual IDL attributes for each CSS property to be implemented on CSSStyleDeclaration. We, as of right now, implement this as an anonymous getter, but this creates some interoperability issues (see bug 700338 for more information). The problem is with dashed properties, as according to the spec (https://drafts.csswg.org/cssom/#dom-cssstyledeclaration-_dashed_attribute) they need both an attribute in camel case and with the dash. WebIDL identifiers are defined as /_?[A-Za-z][0-9A-Z_a-z-]*/ (note that a leading dash is not allowed), so e.g. -webkit-line-clamp would not be a valid IDL attribute.
> 
> (incidentally, our current IDL parser does not allow dashes anywhere in an attribute name, but that's something that can be fixed)

So in order to follow CSSOM spec, our implementation and Web IDL spec need to be updated to allow a leading hyphen in IDL identifier.