It’s a rather common thing to do, and I had to do it recently.

I was blindly assigning bottom: 0 to an element which I wanted to stick to the bottom of its parent.

Turns out I was forgetting 2 things: setting position: absolute on that element, and adding position: relative on the parent.

Example:

<div class="container-element">
  ...
  <div class="element-to-stick-to-bottom">
    ...
  </div>
</div>

.element-to-stick-to-bottom {
  position: absolute;
  bottom: 0;
}

.container-element {
  position: relative;
}