Is there any particular performance or space penalty for elements defined in a schema but not used by any doctype? One of my developers asked this today and I told him no, but then I realized that I don’t really know, that’s just what I suppose.

(The question arose in the context of a discussion about defining a schema of commonly used elements and including it in other schemas, which might not use all of the common elements.)

Despite some (not much) additional memory used for the cached schema’s internal model there is no additional penalty.