Change from

var evt = new CustomEvent( "message" );
to
 var evt = document.createEvent( "Event" );
to avoid error(below) in IE10(10.0.9200.16384) on Windows8

------ >8 ---[Error from IE10 on Win8]--- >8 ------
 SCRIPT445: Object doesn't support this action
 WebMIDIAPI.js, line 271 character 5
------ >8 --------------------- >8 ------