You can learn ANYTHING online.
How WELL, that is the question eh?
If a person has a base core, a solid point of reference AND some training partners that are experienced too, yes they can.
Attending seminars will help too.
BUT that is ONLY if there is a solid point of reference, a solid core to work from.
Learning BJJ BASICS online when you have years in Judo for example.
Learning Shotokan karate online when you have years in TKD for example.
Learning CLF when you have years in Hung Kuen.
Etc...
Psalms 144:1
Praise be my Lord my Rock,
He trains my hands for war, my fingers for battle !