(tested with Chrome Android toggling desktop site)
It's better than, say, Dan Luu's website or any other one that renounces CSS, so viewing it full screen on a large display turns paragraphs into single lines.
That said I imagine that for most Ruby applications the real answer would be "you don't and you don't really need to". The most obvious killer applications for Fibers and FiberSchedulers seem to be web servers (most requests spend quite some time waiting for DB/API calls) and IO-intensive background jobs (also here, the majority of the time is spent waiting for some external server to respond). Fibers all get multiplexed on the same operating system thread anyway, so you don't really increase parallelism by communicating between individual fibers.
If you really wish to use multiple parallel threads in Ruby, Ractors are the way to go or otherwise C extensions that drop the GIL. That is outside the scope of Fibers though.
Just one nit-pick. Content on this page is really hard to read on big screens. This one line of css fixes this.
body { margin-left: calc(50vw - 280px); }
There's little point maximizing a browser on a big screen so I don't. My portal into the web page is roughly square, about as wide as my screen is high, maybe a little bit wider - no more than 4:3 and probably closer to 5:4.
I use a left-aligned tab tree and and other apps, such as console terminals and my note taking app to the left of the browser, to use the space left over.
Wide screen display of mostly text content doesn't make a whole lot of sense.
You probably implemented your own custom scheduler implementation, right?