I want to create a bar that is the size of my parent element, and 2rem wide. So I created an empty div in which I give these css properties, but it doesn't work, more exactly, it's the "height:100%" that is the problem, because if I replace it by "height:2rem" it works fine.
Can you explain me the reason ?
.container {
  background-color: red;
  width: 100%;
}
.bar {
  width: 2rem;
  height: 100%;
  background-color: black;
}<div class="container">
  <h1>Hello world</h1>
  <div class="bar"></div>
</div>