JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" onfocus="invalid()">
<div id="alert">
  That text is invalid!
</div>

CSS

#alert {
  visibility: hidden;
  background-color: yellow;
  width: 170px;
}

JavaScript

var invalid = true

function invalid() {
  if (invalid = true) {
    document.getElementById("alert").style.visibility = 'visible'
  }
}