Most rust web frameworks are server side, and generate HTML, so I’m not sure what you were looking at, but it absolutely exists. Most rust web stuff is server side generated.
Sure, obviously you can generate strings containing HTML and send those. I'm imagining something like the serverside rendering you get with e.g. React. That said, I don't know what specifically the previous poster was looking for.
I'm a bit confused, what's the difference? SSR React also ends up "generating strings containing HTML and sending those."