It would be nice if operating systems would be exactly that, but for some reason "commercial" OS vendors are not able or willing to provide a safe "native" sandbox completely uncoupled from curated app stores.
The ability of a browser to be a document viewer with a very limited layout system should be implemented in an optional layer on top of a bare bones browser.
Again, this should all be provided by the underlying operating systems (click an URL and instantly run cross-platform applications in a safe sandbox), but for some reason we didn't get that. It's the same with Electron apps by the way, those wouldn't exist if operating systems would come with cross-platform UI frameworks that are actually better than the unholy mix of HTML+CSS+JS. But Microsoft, Apple etc... had decades to get their act together without delivering.