Forgive me, but you should more specifically define the terms you use. As it stands, it looks like by "functional enough for people to actually want to use it" you seem to introduce a circular dependency -- as though an OS is useless unless it's easy to port a browser to it.
The GP post mentiones Plan 9, which is a great example here. There are real people who use Plan 9 every day (I have met some of them!) but Webkit will never be portable to that OS because there is no C++ compiler. This doesn't make the OS non-functional, but it does mean step one to porting a modern browser is "port GCC or LLVM," which is a massive task by itself.
Both Firefox and Chrome source code repositories are in the hundreds of megabytes, even without the required host OS dependencies vendored in. The document you linked to recommends leveraging freetype, cairo, and all kinds of other things that would require serious graphics infrastructure of a sort that's just not feasible.
Modern web browsers are keystones that cap off arches built of hundreds of pieces of very specific software. As time goes by, they require more infrastructure, not less -- c.f. Firefox, where to build the latest version you must also have a Rust compiler that targets your operating system. As the checklist of requirements grows ever longer, the candidate operating systems capable of meeting those requirements grows ever shorter.
Five years from now, I predict the only three operating systems capable of interacting with the web in any meaningful sense will be linux, macos, and windows; bsd and haiku et al will have been left by the wayside.
Ten years from now I predict there will be four operating systems, and one of them will be Fuchsia, because only a megacorp like Google has the resources to accomplish that.