Thanks Rob,

With regard to current allowances of implementation-defined features, the choice of schema matters. The "strict" schema actually removes some areas where implementation-defined extensions were explicit in the schema itself. (And, for a different example, I assert that it is a feature, not a limitation, to have omission of table:protection-key-digest-algorithm mean that the hash function that produces the table:protection-key is implementation defined [and also upward-compatible from 1.1 as well].)