How can I highlight squares under atack and show arrows?
There is similiar question here. Is this feature already supproted?
No, that feature is not supported at the chess.SE site yet. To be clear, that feature is built into Andrew's PGN viewer, but that functionality wasn't part of the initial implementation of the viewer at this site. (Similarly, the ability to include comments/annotations alongside the moves is part of the viewer, but not active on the site.) I agree that highlighting and markers would be a nice addition, and I hope we'll be able to get them at some point. My understanding is that just getting the PGN viewer to integrate as seamlessly as the SE staff did was a non-trivial endeavor, so we might not see additional features until the site grows more active. (But that's pure speculation on my part.)