A motor vehicle seat having a structural frame that includes an upper cross-member, and a backrest module having a foam block and a support arranged below the foam block to support that foam block. The seat includes an upper locking element carrying the support and removably connected to the upper cross-member.

A child safety seat includes a backrest and a base. The backrest and the base are detachably connected. The backrest is provided with a headrest that is movable up and down. Side wings are provided at two sides of the headrest. The side wings are connected to the backrest through rotating shafts. Opposite sides of the side wings are provided with slide rails. The slide rails are spiral. The headrest is provided with sliders matched with the slide rails. The slide rails are provided in a same direction as the rotating shafts. The child safety seat has the following advantages. The sliders are slid up and down to unfold or fold the side wings, thereby adjusting a width of the side wings to suit different users. Such a linkage structure occupies less space and spares more space for other components of the backrest.