Does that mean the specification can change at any time?
The specification does not change arbitrarily: we are extremely careful! As parts of the specification mature, and implementations ship, the spec cannot be changed in backwards-incompatible ways (because the implementors would never agree to break compatibility unless for security reasons). The specification is never complete, since the Web is continuously evolving. The last time HTML was described as “complete” was after HTML4, when development stopped for several years, leading to stagnation.