For websites to have dynamic generated content, you need a server-side language which will fetch informations from a database, and generate the page dynamically based on the requested informations.
User request -> Server get data -> Server send data
Latest technologies include AJAX, which uses asynchronous JavaScript requests to interrogate a server-side script, which will fetch the informations and send them back to your page. This for example will allow you to gather informations and update the page in real time, without the need to reload the document. this is the key point of the new web applications you use everyday.
This is also the basis of modern social network websites and web applications.
With XHTML and CSS only you won't be able to create a dynamic website.