Why does 100% not mean 100% height?

Question

I am trying to get some divss to expand to fill the screen, but I am struggling. I have broken down the issue on this jsfiddle.

What I really want to kn

Answer 1

This will work

   body, html {
      height: 100vh;
    }

    aside {
      background: green;
      width: 200px;
      height: 100vh;
    }