The invention relates to a child safety seat (100) comprising an integral harness belt (102) for restraining a child in the child safety seat (100), a belt tension monitor (106) adapted for monitoring a belt tension acting on the harness belt (102), a motorized harness belt tensioner (104) comprising a tensioning element (114) driven by an electromechanical actuator (122) and adapted for adjusting the belt tension acting on the harness belt (102), and a controller adapted for controlling the motorized harness belt tensioner (104) to adjust the belt tension to match a first predefined belt tension and for monitoring the resulting belt tension using the belt tension monitor (106).