Nothing, and you really should, but companies don't because they want to make all modern shiny stuff without putting the effort in to make something that does progressive enhancement properly. That's why contracts for police websites go to companies whose websites look like crap.
It actually doesn't render properly for everyone, and in fact renders incorrectly in chrome: the nav links (Home, Features, Support) overlap with and intersect behind the main content.
Of course, you probably won't notice this unless you're using a dark mode extension like me, because under normal circumstances, those links are invisible, since they render white-on-white.
Also, tables play poorly with mobile and are usually the opposite of responsive.