If you want to see the default browser errors being displayed, for example, those triggered by HTML attributes (showing up before any client-code JS treatment):
<input name="o" required="required" aria-required="true" type="text">
You should use the submit event instead of the click event. In this case a popup will be automatically displayed requesting "Please fill out this field". Even with preventDefault:
$('form').on('submit', function(event) {
event.preventDefault();
my_form_treatment(this, event);
}); // -> this will show up a "Please fill out this field" pop-up before my_form_treatment
As someone mentioned previously, return false would stop propagation (i.e. if there are more handlers attached to the form submission, they would not be executed), but, in this case, the action triggered by the browser will always execute first. Even with a return false at the end.
So if you want to get rid of these default pop-ups, use the click event on the submit button:
$('form input[type=submit]').on('click', function(event) {
event.preventDefault();
my_form_treatment(this, event);
}); // -> this will NOT show any popups related to HTML attributes