This web server was written by Johnny Billquist, and it runs on an emulated PDP-11/74 (under E11). The web server is written in BASIC+2 V2.7, and it uses the TCP/IP stack also written by Johnny Billquist. There is a second web page here as well.
The TCP/IP stack itself is written in MACRO-11, and libraries exist for writing software in both MACRO-11, PDP-11 C, FORTRAN 77, BASIC+2 and other languages on the PDP-11. The TCP/IP stack itself appears as a number of devices in RSX, and access to TCP can also be done though the normal command line tools available in RSX.
Here is a pretty picture of the kind of machine serving this page.
Another fun thing is CGI, for which I have created a testpage.
There is also a second page with some CGI demo code.
There exist one other (known) machine on the internet serving web pages, that is also PDP-11-based, and that is shop-pdp.net
Here is a link to the sources of this webserver.
A couple of services on this machine: