I think 10.3/11 (C++11; no change in N3690) is currently slightly rubbish:

A virtual function declared in a class shall be defined, or declared pure (10.4) in that class, or both; but no diagnostic is required (3.2).

Though the following can be deduced, I think the passage would be clearer and easier to read if it went something like this:

A virtual function declared in a class shall be defined, or declared pure (10.4) in that class, or both; however, no diagnostic is required if neither is true (3.2).

I also fixed the start-of-phrase/post-semicolon conjunction though, admittedly, that is something of a subjective style issue.