If you want to position child boxes at a corner, apply "absolute" and margin% to child box becuase all margins depends on the parent's width, not height. Don't use left%,top%,bottom%,right% because left and right depends on the parent's width but top% and bottom% depends on the parent's height.

Box1 : width:300px;height:150px;
Box2 : margin:10%

parent box's width changes the margin(10%) of child box
Box1 : width:600px;height:150px;
Box2 : margin:10%

parent box's height will not change the margin(10%) of child box
Box1 : width:300px;height:300px;
Box2 : margin:10%