Contenteditable Height transition

Question

I have a contenteditable div which grows as the user types.

I now need to transition the height so that when the user presses Enter, the div would grow s

Answer 1

@keyframes lineInserted {
  from {
    height: 0;
  }
  to {
    height: 20px; /* cons: hardcoded height */
  }
}
div[contenteditable] > div {
  animation-duration: 300ms;
  animation-name: lineInserted;
  transition: height 300ms;
}
div[contenteditable] {
  border: 1px solid black;
  max-height: 200px;
  overflow: auto;
  transition: all 300ms ease;
}

<div contenteditable>
  Testing
  <br/>one two three
</div>

Answer 2

Final solution based on Orland's answer. Thank you, bootstrap!

HTML

<link rel="stylesheet" type="text/css" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css">

<div contenteditable class="form-control">
    Testing <br/> one two three
</div>

CSS:

@keyframes lineInserted {  
    from { height: 0; }
    to { height: 20px; }  /* cons: hardcoded height */
}

div[contenteditable] > div {
    animation-duration: 200ms;
    animation-name: lineInserted;
}

div[contenteditable]{
    height : auto !important;
    overflow: auto;
    line-height : 20px;
}