Different countries have different regulations.
But for the EU they are very concerned with harmonics in the neutral line.
There is a EU standard IEC1000-3-2 which deals with this topic. It will explain in excruciating detail what the maximum levels should be.
I read that standard many years ago, and cannot recall much other than the measurement bandwidth is 39 harmonics for 50 Hz, and 33 harmonics for 60 Hz. In other words, a measurement bandwidth of around 2 Khz.