JSFiddle - React, Tailwind, and code Playground

HTML

<input type="text" name="phone" id="phone">

CSS

#phone.error{
  border: 1px solid red;
}
#phone.error:after{
  content:'Введите телефон';
  display: block;
  color:red;
  font-size:12px;
}

JavaScript

$('input[name="phone"]').bind('focusout', function(){
  if($(this).val().length === 0){
    $(this).addClass('error');
    return false;
  }else{
    $(this).removeClass('error');
    return true;
  }
});