How to make something like float: down with relative position?

Question

I'm trying to move the element to the bottom of its parent but only way to do that is to make it positioned absolute but i want it to be relative.

#output{
  height: 100px;
  background-color: red;
}

#inner{
  height: 30px;
  background-color: blue;
  /*float: down*/
}

<div id="output">
  <div id="inner"></div>
</div>

Answer 1

#output{
  height: 100px;
  background-color: red;
  display: flex;
  flex-direction: column;
  
}

#inner{
   margin-top: auto;
   height: 30px;
   background-color: blue;
   /*float: down*/
}

<div id="output">
  <div id="inner"></div>
</div>