A form's "action" and "onsubmit": Which executes first?

I'm trying to debug a webpage and I see a form element whose opening is

<form name="aspnetForm" method="post" action="default.aspx" onsubmit="javascript:return WebForm_OnSubmit();" id="aspnetForm">

Having only a base knowledge of web forms, I'm wondering what the order of execution is for the action and onsubmit.