http://en.wikipedia.org/wiki/MIDI
Computers and MIDI is 30 year old technology.
In 2006 Microsoft changed the rules for HTML code, ActiveX and Internet Explorer. None of the example links use "modern" code. Other browsers may work with that older code, however.
None of the example code "calls" the QuickTime browser plug-in (ClassID number) so only a handful of page viewers would even hear the .mid file while others may get a browser plug-in "error" message.
If you right click on the link I posted above and open it in a new browser window you'll see that the .mid file is downloaded. The downloaded file will play in QuickTime (any version except QuickTime X). QuickTime X is Mac only software.
I no longer see the .mid file extension suppported by the QuickTime browser plug-in (Safari Help menu / Installed Plug-Ins). This probably sends the message that browser support (not computer) for .mid files is coming to an end.
I wonder what would happen if the page authors changed the .mid file extension to .mov (page code and file name)? I don't have a Web server to test this idea.