Hmm I believe for an online session the sub has a greater responsibility to his/her own safety because the Dom cannot see firsthand what is going on on the sub's end. Theres no excuse here, dont be a dummy when playing around, especially if theres no one around to help or make suggestions.

In real time the responsibility is shared because the sub Must communicate his/her needs appropriately and the Dom Must be able to Read her body language and be careful Not to get carried away during a session. Or sometimes the sub is vulnerable, sometimes gagged, tied, inable to respond in a necessary way and the Dom is more obligated to know what he's doing in order not to harm the sub to the point of damaging him/her mentally. Be an adult. This isnt child's play :P

^_^