I think a web browser is particularly suited to a large number of developers, like an OS, since it has so many components from different threads of software dev

I think a web browser is particularly suited to a large number of developers, like an OS, since it has so many components from different threads of software development (networking, parsing, JS + WASM Engine, layout & graphics, text shaping, font rendering, GUI). Further, the top level components are easy to decompose again (layout engine, composite engine, SVG renderer, CSS effect renderer - for example). A modular browser would be nice - where custom components can be swapped in for certain functionality (layout, networking, JS engine, CSS engine, renderers for various MIME types).