I have posted some teardowns of serveral electronics devices and have some notes on their electrical safety. Examples of ones with questionable safety were this is tiny USB charger and this USB charger. The later USB charger posting has also example of power supply that seems to have properly built safety in it.
But how is really safe power supply built ?A very recent EEVblog #659 – Medical Plugpack Teardown video gives some answers to that. It shows what is inside an IEC60601-1 medical class 5V mains power adapter (German FRIWO 5V FW7555M/05 brand medical grade isolation unit). It also compares the medical power supply construction to a consumer brand power supply. Besides what is in the circuit board the video take a look at the transformer construction.