Re: Plugin: bbMenu 0.1 beta
Syntactically speaking: only if a page does not validate is it “wrong”.
Semantically speaking: if a page uses
id to encode semantic meaning then it is syntactically correct, but semantically naive (which for a “semantic personal publishing platform” is sub-optimal).
There are possibly places where WP gets semantically wrong, but, a lot (possibly all) of this is down to theme and plugin designers who are often unaware of the semantic issues.
Anyhow, your description in the opening post (two items returning “current” as their id) is something that will create a non-validating document, so it would be syntactically wrong.
id is for addressing elements,
class is for describing them.