How to expand textarea width to 100% of parent (or how to expand any HTML element to 100% of parent width)?

<div>
  <div style="width: 20%; float: left;">
    <p>Some Contentsssssssssss</p>
  </div>
  <div style="float: left; width: 80%;">
    <textarea style="width: 100%; max-width: 100%;"></textarea>
  </div>
  <div style="clear: both;"></div>
</div>