We strive for interoperability with both our own, and our upstream, communities.
We did a merge from the main MySQL branch for every new MySQL release or when there was some critical bugfix applied to the main branch.
Since MariaDB 10.0, MariaDB includes backported features from MySQL as well as entirely new features not found anywhere else, but does not necessarily include all MySQL features.
We strive to keep our main trees as free from bugs as possible. It should be reasonably safe to pull from our trees at any time.