How are you (and everybody),

I have a question somewhat peripheric to WB, but an answer would be appreciated.
Here is one of my website page :
http://www.khenpo.fr/ktgr_bio2.html
Previously, if you mouseovered on a in red-brick colored word in the text, a tooltip would appear.
Now, without any change by me, in Chrome, Edge,.. it doesn't work at all. In Firefox, the frame of the shapes appears, but it is transparent, with nothing inside.
Any advice would be appreciated.
Thank you so much.