Swi-Prolog, a popular, free and open source implementation of Prolog, has good support for web development. The Swi-Prolog website itself is written using Swi-Prolog's http libraries.

There are a couple of tutorials listed under Tutorials > Web applications on the website. Probably the place to start is:

Tutorial — Creating Web Applications in SWI-Prolog

http://www.pathwayslms.com/swipltuts/html/index.html