menu items in navigation are underlined by default in some browsers

I am currently working on a wordpress page with the zerif-lite theme.

I face an issue where the navigation items (aka menu items) are displayed differently on different computers and I don’t know the reason for that behavior. I did no CSS-customization or whatsoever on the menu items.

For my friend the menu items are underlined by default (that means: no menu item is active or clicked).
This is how it looks when he just loads the page: here
This is how it looks when he clicks a link: here

For me, the links are not underlined by default and I think this is the correct behavior. This is how it looks: here

Does anyone know what the reason for the default-underlining is? How can I remove it? Currently I am not able to replicate the issue, unfortunately. So I have no clue which CSS is causing this