In particular, some recent research papers have shown that functional programming works very well for this domain (use of continuations or closures, continuation passing style ...). Accordingly, Ocsigen considers each URL as a function taking arguments and producing a web page. Clicking on a link or a form triggers execution of this function.
Another significant issue addressed by Ocsigen is the generation of valid xhtml pages. The web relies on norms edicted by the W3 consortium, and the only means to ensure universal availability of information is to respect these standards. Today, very few web sites are fully standard compliant. Ocsigen uses an advanced type system that guarantees that your web site will be valid (or very close).
Link from Phil Wadlers blog.