Is it possible to do Web form without using server control or set runat attribute on html control? How do you call the code behind function?
You can't call codebehind functions without a runat="server" tag at a minimum. If you created a Web Service instead, you could create a pure html/javascript page that interacted with the server through AJAX. These are your only two options to use ASP.Net as far as I know.
Yes, it is possible to do this. The form with runat server is only needed if you use postbacks and server controls.
If you do not use server controls you should be able to add forms to the page that POST to other pages (it can even post to itself). In your page_load you will be restricted to using the normal request.form and request.querystring to retrieve form values, but you should be able to call other methods on the page.
If you are familiar with classic ASP, you can do the same thing with asp.net.
Also, take a look at the asp.net MVC framework (http://www.asp.net/mvc). It allows you to use asp.net without using webforms.
You can use a HTTPHandler for barebones ASP.NET.
You won't have a markup file, you'll just have a class that runs and exposes you to HttpContext for writing out to the HTTP stream.
http://msdn.microsoft.com/en-us/library/f3ff8w4a(VS.71).aspx
In fact, HttpHandlers are the building blocks of all .NET web frameworks.