We investigate the problem of querying (regular) sets of XML documents represented with tree automata and we consider $n$-ary tree automata queries whose expressive power captures MSO on trees. Because finite automata can represent infinite sets of documents, we propose the notions of {\em universal} and {\em existential} query answers, answers that are present resp. in all and some documents. We study complexity of query answering and show that computing existential query answers is in PTIME if we assume the arity of the query to be a fixed parameter. On the other hand, computing universal query answers is EXPTIME-complete, but we show that it is in PTIME if we assume that the query is fixed (data complexity). Finally, we argue that the framework captures problems central to many novel XML applications like querying inconsistent XML documents. In particular, we demonstrate how to use our framework to compute consistent query answers in XML documents that do not satisfy the schema. This solution significantly extends our previous results in this area.