Validate an update form with JavaScript

I have an update profile form. When a user logs in he can update his profile information. When the user goes to this page the data is pre-populated from the database and the user can update the data and hit submit which updates the data and saves it.
What I am trying to do is validate the form using JavaScript. I put runat=server on all the HTML controls and in the button (input type=image).
Code snippet:
<form method="post" id="Form1" runat="server" autocomplete="off" onsubmit="return validate_form(this)"> <table> <tr> <td>First name</td>
<td align="left"><input border="0" size="46" name="first_name" id="first_name" runat="server"> *</td> </tr> <tr> <td>Last name</td> <td align="left"><input border="0" ...