When you are using %
for width, or height, the 1st question you should ask is that 80%
of what? So you also need to apply height to the parent element, so assuming that this element of yours is inside the body tag, you need to use this in your CSS
html, body {
height: 100%;
}
So now your div
element will be 80%
of 100%
Side Note: Also when you are dealing with absolute
positioned elements, you may come across a scenario where your div
won’t exceed the current viewport height, so in that case you need to have min-height