<form id="userForm">
    <input type="text" name="username" placeholder="Имя">
    <input type="text" name="email" placeholder="Email">
    <input type="text" name="email" placeholder="Повторите Email">
    <button type="submit">Отправить</button>
</form>

<script>
    const emails = document.getElementsByName('email');
    
    for (let i = 0; i < emails.length; i++) {
        emails[i].addEventListener('blur', function() {
            if (this.value.length < 5) {
                this.setCustomValidity('Email слишком короткий');
            } else {
                this.setCustomValidity('');
            }
        });
    }
</script>