_Raised by Boris Zbarsky:_
In "web" specs and libraries it seems to be the common style to put
options objects as the last argument of a method.
I am given to understand that "Node" style is to put the callback, if
any, as the last argument, and that json-ld is following that style
here. But it's not obvious why that's preferable to following "web"
style for this particular specification.
-Boris