What operating system ran the world's first Web server? (Hint: It wasn't Windows, Mac or Linux!)

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On Aug. 6, 1991, Tim Berners-Lee first shared his ideas for the World Wide Web via the Internet. (Yes, the Internet and the World Wide Web are different things, though the distinction is more nebulous these days.) Berners-Lee actually waited several months before sharing his Web concepts with the larger online world; history's first Web server went live from CERN on Christmas Day 1990.

In the interim, Berners-Lee spent time perfecting his World Wide Web software, including the concepts of a server and a browser. He did all this on a then-outdated workstation running an operating system that was neither Windows, Macintosh, nor Linux (the last of which wasn't even invented yet).

What operating system ran the world's first Web server?