Hi,
There is no perfect CSS method of targeting browsers depending on the medium they run on (mobile/table/desktop). Generally, CSS is able to target devices depending on screen size: https://stackoverflow.com/questions/6370690/media-queries-how-to-target-desktop-tablet-and-mobile
Browser detection is usually done with JS or through code, and even then it isn’t perfect since browser useragent strings can be manipulated.
For a mobile device in landscape mode you’d want to target a screen size between the largest portrait mode you want the menu to remain visible on and the smallest desktop you’d also want a menu visible one. This approach is bound to overlap some less common situations (like tables in portrait mode or ultra portable laptops).