I also think OS-specific design is perfectly appropriate for a lot of use cases. I use "font-family: system-ui, sans-serif" on my resume page, for example, because I know that the default system fonts look fine (by design) on any screen and a fast, lightweight page is more important to me than a specific font presentation. If you're creating a user interface with really tight tolerances on element size or appearance, different strategies might be more appropriate.