From the spec (http://dev.w3.org/html5/webdatabase/):
This specification has reached an impasse: all interested implementors have used the same SQL backend (Sqlite), but we need multiple independent implementations to proceed along a standardisation path. Until another implementor is interested in implementing this spec, the description of the SQL dialect has been left as simply a reference to Sqlite, which isn’t acceptable for a standard. Should you be an implementor interested in implementing an independent SQL backend, please contact the editor so that he can write a specification for the dialect, thus allowing this specification to move forward.
That is an interesting predicament. Though, off the top of my head, I can’t think of another SQL backend that a browser implementor might be interested in using. Most of the others just feel too heavyweight for the purpose of this exercise. I might be wrong though.
I guess they could always just refer to the standard SQL dialect ;)