How to do a specific menu hover?

How to do a specific menu hover?

I have been looking through the code here  https://devitems.com/html/apnew-preview-v2/apnew1/index.html to try and discover how they are doing this hover transition on the menu. Can anyone tell me how they do this or how to do it in general? I thought it was CSS transition using the Begin attribute, but that doesn't seem to do it.