Problems with bubbletip extension on Internet Explorer 11

Hello here,
I am struggling to understand why the following page shows correctly bubble-tips when you move the mouse over the two links if you open it in any browser but Internet Explorer 11:

http://www.virtualsheetmusic.com/ziolo1.html

Any ideas? The browser seems to ignore the CSS or what?

Any thoughts are very welcome!

Thank you in advance.

Fab.