WinUI essentially webpage

I don't even mean the modern stuff. Even fairly old Windows apps can do fractional scaling just fine. WPF does it without a hitch, and it dates back to 2006. Even many WinForms apps can do it, and then of course Qt etc.

But even disregarding all that - if such scaling can be done without problems with webpages, then clearly there's no technical impediment to doing the same elsewhere. It's not like HTML is magic.