I noticed this tag being removed from images that clearly had a hand on someone's head and was a little confused... but then I checked the wiki entry and there is a claim there that petting should be used instead whenever it's "non-sexual". However, all of the relevant edits on the wiki were done entirely by one user, memegui. Any thoughts on how these tags are supposed to be used? Since most images on Danbooru are static, "petting" doesn't really differ much from "hand_on_head" in my mind.
Updated by jxh2154