ASP.NET Development

hi, i wanted to know the exact use of runat="server" which is being used is ASP.NET why we every time need to use it, even when i want to do the work at client side? How to do simple processings like displaying some message ...

i use WebRequest to read response from the URL i request. the HTML returned contain an HTML buttons which do post back to server, after many time "HTTP 403.9 - Access Forbidden: Too many users are connected". this is snapshot of the code: ...