1967:
https://en.wikipedia.org/wiki/Hypertext_Editing_System was internet in some sense, too, even though it was centralized and, one might say, not even ‘net’, let alone internet.
A year later, https://en.wikipedia.org/wiki/File_Retrieval_and_Editing_Sys... got a bit more ‘net’ (https://www.brown.edu/news/2016-05-23/hypertext has a movie about it that is worth watching but unfortunately doesn’t show how it worked). At least, it supported multiple terminal types. That page also claims “FRESS still runs on the current Brown mainframe.” Iet would be amazing if that were true. Software that’s over 50 years old.