how to make div item as tall as root parent item code example

Example 1: css fill parent height

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

#root {
  width: 100%
}

Example 2: css fill parent height

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