JSFiddle - React, Tailwind, and code Playground

HTML

<html>
<head>
<script type="text/javascript">
      function validateForm(oForm){
    var els = oForm.elements;
    for(var i = 0; i < els.length; i++){
       if('string' === typeof(els[i].getAttribute('data-message'))){
          return valEl(els[i]);
       }
    }
} 

function valEl(el){
    var method = el.getAttribute('data-valMethod');
    if('req' === method && (el.value === null || el.value === '')){
           alert(el.getAttribute('data-message'));
           return false;
    }
} 
</script>
</head>
<body>
<form name="myForm" action="#" onsubmit="return validateForm(this)" method="post">
First name: 
    <input data-message="First name must be filled out" data-valMethod="req" onchange="return valEl(this)"; name="fname"><br />
<input type="submit" value="Submit">
</form>
</body>

</html>