While most pundits regard the deep, formal semantics promised by the likes of Powerset as not important to search I feel that I am personally finding search dead-ends in my long tail queries that clearly indicate the need for this type of feature. I will commit the sin of using a single example to support my point.
Perhaps there are no actual pages online that will answer my query and the incorrect (they are not alternate, they are wrong) interpretations are backfilling. But this is useless to me as a user. I'd rather know that the engine understands what I'm looking for and can tell me 'you won't find what you are looking for in our index.'
Update : here is a suitable hit for this query : element should not contain script tag from the w3c's feed validator documentation.