I wrote a web app for a client back in the late '90s that is still in (heavy) use at the company. It was actually a "Classic ASP" app and they kept one old PC around to act as the server for it for a couple of decades (they eventually replaced that with a virtual machine and the app is still going). The output is straight HTML + CSS so they've never had any problems using it with progressively more modern browsers. Ironically, this app is a front end sitting atop an unbelievably clunky mainframe application that dates to the 1970s, so my app's continued existence means that mainframe application is still running as well.