It is a real, functioning emulator of the Macintosh Plus from 1986. See the webpage[1] of the James Friend, who ported the emulator to JavaScript, as well has his posts[2][3] explaining more detail behind the project.
(As an aside, the Readme file probably didn't work because no text editor was installed - the items in the HyperCard stacks collection typically only include the OS, HyperCard and the stack itself. If you try opening the stack it should work just fine.)
Yeah it's impressive and also sad in a way, greatest thing now is later obsolete, that's life I guess.
I remember seeing a video of someone loading Google with an old Macintosh(tan tube type) it took a while, but it eventually loaded, felt like time traveling or something.
[1] https://jamesfriend.com.au/pce-js/
[2] https://jamesfriend.com.au/why-port-emulators-browser
[3] https://jamesfriend.com.au/porting-pce-emulator-browser
(As an aside, the Readme file probably didn't work because no text editor was installed - the items in the HyperCard stacks collection typically only include the OS, HyperCard and the stack itself. If you try opening the stack it should work just fine.)