<form action="<?=POST_FORM_ACTION_URI?>" method="POST" id="callback" name="callback">
<div>
<div>Ваше имя<span>*</span></div>
<input type="text" name="author" value="" >
<div class="form__error">Нужно заполнить!</div>
</div>
<div>
<div>Номер телефона<span>*</span></div>
<input type="text" name="phone" value="" >
<div class="form__error">Нужно заполнить!</div>
</div>
<div class="form__buttons">
<input type="submit" class="form__submit" name="submit">
</div>
</form>
<script>
$(document).ready(function() {
$('form').on('submit', function(){
var formID = $(this).attr('id');
var formNm = $('#' + formID);
if (formID == 'callback') {
$.ajax({
url: '/include/send_callback.php',
type: 'post',
data: formNm.serialize(),
success: function(data){
$(formNm).html(data);
}
});
return false;
};
});
});
</script><div class="form__error">Нужно заполнить!</div><div class="form__error is active"> 
<input id="id1" type="number" min="100" max="300" required>
<button onclick="myFunction()">OK</button>
<p id="demo"></p>
<script>
function myFunction() {
var inpObj = document.getElementById("id1");
if (!inpObj.checkValidity()) {
document.getElementById("demo").innerHTML = inpObj.validationMessage;
}
}
</script>