differentiele probe
Testec differential probe and similar are actually not
isolated probes rather than differential amplifiers with good common mode rejection. The differential amplifier is still referenced to the output (oscilloscope) ground and uses compensated resistive voltage dividers to achieve the common mode and differential voltage range.
Probes with true optical isolation are also manufactured, but only used for special purposes as HV measurements. By state of the art, you'll find now battery operated and WLAN remote controlled oscilloscopes in HV experiments.
To my opinion, the said differential probes are a must for most power electronics measurements, although they still suffer from common interferences in fast switching circuits. Keeping this limitations in mind, they allow a live view at power electronics.
See a Testec probe PCB detail view for reference: