Interesting note indeed. I remember there being a different Minecraft Classic mode from somewhere around 2009-10 that just ran on Java and you could play it through a browser (Java applet I guess) or download a Java client. The server was in Java too, of course. So perhaps they decided to rewrite it in JS?

I used to be an admin on one server and split my time between building stuff and banning griefers/undoing the griefing on the main worlds to make everything pretty again. Great memories.