JSFiddle - React, Tailwind, and code Playground

HTML

<form action="new_user4.php" method="post">
<div id="usernameformitem">
  <p>Username:
    <input type="text" name="username" value="" />
 </p>
 </div>

  <input type="submit" name="submit" value="Create User" />
</form>

CSS

.formerror { border:1px solid red }

JavaScript

function myFunction() {
  document.getElementById("usernameformitem").className = "formerror";
}
window.onload=function() { if (myFunction) myFunction() }