R’ Shlomo Kluger held that a Kippah has to cover Rov of the head. The first T’shuvah in the Igros Moshe very strongly disagrees with that P’sak. R’ Moshe Shternbuch tries to make a Chiluk to be M’kayeim the P’sak, but I think the Olam has accepted Pashtus like R’ Moshe (Feinstein).
R’ Shlomo Aviner in a T’shuvah says it has to be visible from all sides.