JSFiddle - React, Tailwind, and code Playground

HTML

<div id="testDiv">
  <span id="testSpan">Hello</span>
  <textarea id="testTextArea"></textarea>
</div>

CSS

div
{
    height: 300px;
    width: 400px;
    border: 1px solid red;    
}
span
{
    display: block;
    background-color: red;
}
textarea
{    
    background-color: blue;
}

JavaScript

function test()
{ 
    var div = document.getElementById("testDiv");
    var span = document.getElementById("testSpan");
    var textArea = document.getElementById("testTextArea");
    var height = div.clientHeight - span.clientHeight;
    textArea.style.height = (height - 5) + "px";
    textArea.style.width = (div.clientWidth - 5) + "px";
}

test();