This is an archived snapshot of W3C's public bugzilla bug tracker, decommissioned in April 2019. Please see the home page for more details.
See above URL for construct that allows <BR /> to be passed as valid as at Sun Aug 19 12:06:58 BST 2007 it appears to me you intend to only allow <br></br> as valid in other contexts.
I found the validator comments confusing till I tried some extra markup that got me an explanation I followed.