first-child This selector only choose the define element if the the element is the first child of the parent. If the define element comes second position then this selector can not select that element.
First-of-Type This selector chose the define element if the element comes 2nd, 3rd or 4th any place but have to be the first of it’s type.
Box-sizing: border-box Size of box does not exceed beyond specification due to padding and border.
not not implementing something with the target, but rest of the elements.
border-collapse: collapse Gap between cells of table becomes invisible with this.