How to stretch div height to fill parent div - CSS

Answer 1

Suppose you have

With normal CSS, you can do the following. See a working app https://github.com/onmyway133/Lyrics/blob/master/index.html

#root {
  position: absolute;
  top: 0;
  left: 0;
  height: 100%;
  width: 100%;
}

With flexbox, you can

html, body {
  height: 100%
}
body {
  display: flex;
  align-items: stretch;
}

#root {
  width: 100%
}