Height = 20vh. This div occupies 20% of the window height. It does not depend on the parent's height. It only depends on the browser window height.
Height = 20vh. This div also occupies 20% of the window height. It does not depend on the parent's height (which is 25vh). It only depends on the browser window.
Height = 80%, but it does not work. Percentage only works if parent height has been defined.
Height = 80%. It works because its parent element has its height set to 30vh. So, its height is 80% x 30vh = 24vh.