Currently, the various dictionaries used by WebCrypto mark members as nullable ("?") when they are not required, and required members are unadorned.  This is backwards, since dictionary members are optional by default.

Instead, the dictionaries need to mark required elements with the new WebIDL "required" keyword, and optional elements should be unadorned.  This will enable the WebIDL layer to reject dictionaries with missing required members, and non-required members will have to be considered nullable.

http://dev.w3.org/2006/webapi/WebIDL/#idl-dictionaries