I had checked youtube.com and stackoverflow.com. And found both of them can not log out if the network is unreachable.
I think this is a security risk, right?
Please image a situation below:
I visited this website on my friend's computer. And before I leave, I clicked the logout button but the network is down in that time. So I go home and thinking that I had logged out to succeed. But on another day, my friend opens his computer to start to visit stackoverflow and found my account still there.