The decision was made to not rely on OS detection or a user configuration to deliver only Linux, Win or macOS wiki content.
That is because browser detection would deliver the wrong content if you ran Gramps on one computer but browsed on another. (I operate on Windows but browse on a smartphone.)
And depending on a user configure means we’d need EVERYONE to create an account & login. Right now, only WikiContributors log-in. Everyone else is an anonymous visitor.
The other approach that has been rejected is collapsible/hidden sections for each OS. The logic being that collapsible sections are not compatible with webpage printing or PDF generation.
We only have 3 pages that are radically different for Users of differing OSes: the downloads, the User Directory & Keybindings. (Keybindings can remain combined with the 3 OSes as columns in a table.)
Downloads & User Directory could be changed to landing page that explain the basic concept – then branch to 3 separate pages, each specific to an OS.