Est-il prudent d'activer une carte mère en dehors de son boîtier?

I have a new motherboard which needs a BIOS update to support a new CPU. Thankfully, I have an old (compatible) CPU handy. My plan is to install the old CPU, update the BIOS, then install the new CPU. I would prefer not having to mount the motherboard in a case just to do the BIOS update bit.

Is it safe to power up the motherboard outside of its case? My understanding is that the case provides grounding for the motherboard. Could the lack of grounding be an issue?

Yes, it should be safe. Just be sure to put your motherboard on something non-conductive, like a cardboard box, and it should not touch anything that conducts electricity, including your main computer case. I have done this a few times. If you stop by almost any computer shop, technicians do this sort of thing routinely.

Yes, you can power up the motherboard outside of its case. Just take some precautions, like laying a piece of cardboard underneath the board, and you are good to go.

Also, the human body contains a static charge, so ground the static by touching a grounded appliance or wiring a ground circuit. Static charge in the human body might damage the sensitive electronic components on the motherboard.