Complex sentence: Font families should not be explicitly defined in an article, with the exception of PUA characters (next section), because this interferes with Wikipedia's flexibility, and it is impossible to foresee what fonts will be installed on a user's computer.
Split into simpler sentences: Font families should not be explicitly defined in an article, with the exception of PUA characters (next section). On the other hand, it is impossible to foresee what fonts will be installed on a user's computer.