[heycam/webidl] Grammar does not allow non-readonly regular attributes in mixin (#702)

In [interface mixin](https://heycam.github.io/webidl/#idl-interface-mixins) section, we enumerate [regular attributes](https://heycam.github.io/webidl/#dfn-regular-attribute) in *MixinMember*.
In [grammar](https://heycam.github.io/webidl/#idl-grammar) chapter, on the other hand, it does not allow [*ReadWriteAttribute*](https://heycam.github.io/webidl/#index-prod-ReadWriteAttribute), which is a kind of regular attribute, as a member of [*MixinMember*](https://heycam.github.io/webidl/#index-prod-MixinMember).

-- 
You are receiving this because you are subscribed to this thread.
Reply to this email directly or view it on GitHub:
https://github.com/heycam/webidl/issues/702