I am new at C#.Net and i have one question that i couldn't manage to find on the internet. When should i use a classic combination of html + javascript + css instead of using an aspx page with a code behind? As i experienced since i started .net , i found that aspx pages and code behind is a huge ease for developers. I didnt need any piece of javascript code since i started. There must be something wrong. I think i am missing a point. Can you answer my question and tell me some examples that i must use html+javascript+css instead of aspx + aspx.cs or vice versa?? Have a nice day.
Javascript is a client side technology, running only in the browser, whereas ASP.NET runs on the server side. These allow you to achieve different and complementary things.
With a classic server side language, any user interaction that you want to respond to must typically be posted across the internet from the browser to your server. It is then processed by the server, which responds with a new page for the browser to load. This generally means that the response time for the user is slower, though you will have access to a much richer programming environment on the server.
With a client side language, everything is processed on the browser. This allows for faster feedback to the user, though at the expense of working within the much more restricted programming environment that the browser gives you, and with no access to stuff your application may depend on, such as your database.
Of course, the lines are blurred somewhat when you make an AJAX request (usually a call written in Javascript that makes a request to the server, receives the response, and updates the page dynamically).
You mention that you have not used any Javascript so far. Perhaps as a starting point you'd like to investigate validating user input on the client side? This way, errors are caught and reported to the user immediately without the cost of the round trip to the server. http://www.tizag.com/javascriptT/javascriptform.php
Both client side and server side technologies can be powerful and useful. Use a combination of them both to give the best experience for the user.