Any Electrical device should be plugged in with the cord being at the back or it’s a potential tripping hazard, so that is what is meant by operating it (safely). Who would want the cord hanging off in front of a table or shelf anyways? it’s a long cord most of us are trying to hide!
I’ve not seen any mention of glass Windows but .in my own opinion Windows should be avoided because of sunlight heating up the unit or condensation getting onto it, as well as most air vents/heating vents/units being placed near Windows. None of my Window ledges would fit it. A window that is in hot blazing sun would not be ideal. It has a lot of bass and thin glass would rattle! A window is only no good for these reasons.
Apple recommends:
Place HomePod on a solid surface, anywhere in the room. Make sure that it's 6 - 12 inches away from any wall or corner with at least 6 inches of space around it. HomePod senses where it's placed, then automatically analyzes the acoustics in the room, steering music in the optimal direction.
https://support.apple.com/en-ca/HT208241
I can’t find where it was mentioned to avoid heat sources etc, it would be in the operating it safely section.
i don’t really adhere to any of these suggestions apart from having my cord at the back because that one comes down to common sense..