Video is nearly 3 years old now, but I think it’s worth watching. Her presentation starts at around 2:30.
Basically, she explains how Redbean, a tiny (~450kb) and very fast C http server, works and how the same executable can be used to deploy it on most operating systems (she starts explaining that around 14:30)
Justine is also the mind behind Sector LISP, Lambda Calculus in 383 bytes, considerable optimizations to LLamaAI, plus several other things.
What she’s accomplished is, to me, astonishing and an impressive intellectual feat. I believe there’s a pretty big caveat that it only works on x86_64, i.e. not ARM. The latter being fairly popular and not uncommon.
It wouldn’t damage the impressiveness of her achievement to say “most OSes on x86.”