JSFiddle - React, Tailwind, and code Playground

HTML

<form>
	<h1>Zwei verschiedene Werte eingeben</h1>
    <p>A: <input type="text" name="alpha" /></p>
    <p>B: <input type="text" name="beta" /></p>
    <p><input type="submit" /></p>
</form>

CSS

[type=text]:valid {
  box-shadow: 0 0 1em lime;
}
[type=text]:invalid {
  box-shadow: 0 0 1em red;
}

JavaScript

var $inputs = $('input[type=text]');
$inputs.on('change keyup', function(){
  if($inputs[0].value == $inputs[1].value){
    $inputs[1].setCustomValidity('Zwei _verschiedene_ Werte bitte!');
  }
  else {
    $inputs[1].setCustomValidity('');
  }
});