JSFiddle - React, Tailwind, and code Playground

HTML

<form id="form" onsubmit="return(login())">
    <input type="checkbox" id="cbxActivarCustom" /><label for="cbxActivarCustom">Activar mensajes custom</label><br />
<input name="username" placeholder="Username" required />
<input name="pass"  type="password" placeholder="Password" required/>
<br/>Remember me: <input type="checkbox" name="remember" value="true" /><br/>
<input type="submit" name="submit" value="Log In"/>

JavaScript

$(document).ready(function() {
    var elements = document.getElementsByTagName("INPUT");
    for (var i = 0; i < elements.length; i++) {
        elements[i].oninvalid = function(e) {
            if($('#cbxActivarCustom').is(':checked')) {
                e.target.setCustomValidity("");
                if (!e.target.validity.valid) {
                    e.target.setCustomValidity("Mensaje custom de Validacion");
                }
            }
        };
        elements[i].oninput = function(e) {
            if($('#cbxActivarCustom').is(':checked')) {
                e.target.setCustomValidity("");
            }
        };
    }
})