Some emulators even emulate the sound of the physical fdd :)
Also, I have secret plans to use the progress-bar metaphor as an interface for accessing past game states. Imagine if you started playing Pacman, and the progress bar started filling up. Then you click in the middle of the filled-in bar to jump back to that place in time...
The progress-bar-as-rewind idea is very cool, too.
You can record how many frames or cycles it takes for the first thing to appear on the screen.
Since the emulator is deterministic and you are always starting from the same state, it will always take the exact same amount of emulated time (for each version of the emulator). So your progress bar is simply a measure of emulated time.
Or take it a step further, and store a savestate from the moment the first thing appears on the screen, and the user's emulation will start from there instead.
This would be an interesting area of research. Once we fire up the emulator, it starts drawing to its screen buffer (which is an HTML5 canvas). What you see is presumably exactly what you would see on the original machine (sadly, I never had an Amiga myself). You see the workbench, possibly some messages in it from the software, then the software takes over the screen and starts drawing pretty pictures. In between, a lot of these things seem to blank the screen, turning it black or grey, or filling it with noise.
Providing meaningful and accurate feedback on top of that would be a fun application of computer vision, but it could turn out to be a difficult problem in the general case.
Thanks for mentioning the fullscreen button, I hadn't noticed that it was broken; I'll see if I can fix it. Muting is a horror show, because MAME and DosBox don't yet support it. SAE does though, so we'll work on it next.