Currently we allows anything including numbers and strings for extended attribute rhs:

... but the spec says:

The ExtendedAttribute grammar symbol matches nearly any sequence of tokens, however the extended attributes defined in this document only accept a more restricted syntax. Any extended attribute encountered in an IDL fragment is matched against the following five grammar symbols to determine which form (or forms) it is in:

Grammar symbol	Form	Example
ExtendedAttributeNoArgs	takes no arguments	[Replaceable]
ExtendedAttributeArgList	takes an argument list	[Constructor(double x, double y)]
ExtendedAttributeNamedArgList	takes a named argument list	[NamedConstructor=Image(DOMString >src)]
ExtendedAttributeIdent	takes an identifier	[PutForwards=name]
ExtendedAttributeIdentList	takes an identifier list	[Exposed=(Window,Worker)]

rhs should only allow 1) a named argument list 2) an identifier or 3) an identifier list.

Related to https://github.com/w3c/respec/issues/1760