This is one of the most remarkable uses of in-browser JavaScript I have ever seen. It is an IBM PC emulator, and it really works. Actually, I think it is a fair bit faster than my first (mid-80s) PC and it certainly looks a lot better on my modern screen. Watch out for the incredibly piercing beep as it starts up and each time you get an error or warning message. I recommend keeping the volume down.

For a truly meta-experience, the virtual floppy disk authentically includes the BASIC interpreter. So, you can run an interpreted language - JavaScript - in your browser to run a virtual machine on which you can run another interpreted language, BASIC. It even allows you to save your work, and the state of the machine. It conveniently includes a few virtual keys at the bottom of the screen so you can stop an out-of-control program etc. I needed that as, of course, the first thing I typed when I started the interpreter was:

10 PRINT "Hello world"

20 GOTO 10

RUN

I once thought that was the coolest thing. This, though, is possibly just a bit cooler.