Our 10.5 month old does this, she can go onto the chair, onto the narrow arm of the chair and onto the sideboard. I have no idea how to stop it though, she can’t talk or walk yet. She understands no, that is the funny gamethat means keep doing it while laughing uproariously.
I did teach her to go down stairs safely (backwards, rather than hurtling down head first ), that was at about 8 months, did that by picking her up every time she went for a stair dive and turning her round. Now she heads for the stairs and does a little 3 point turn before going down them.