Web browsers can render more than just HTML, can't they? Plain text at the very least, many do PDF as well.

That function really seems like it should be in the browser anyway. The server serves, the renderer renders.

if the server side file format has something semantically similar to #include then your lovely simple binary model fails (without browsers having the same)

HTML has this, yet browsers seem to cope...?

nginx has a lot of modules, this one can render markdown apparently

https://nginx-extras.getpagespeed.com/modules/markdown/

Especially older servers have quite a lot of those types of capabilities, rendering directories to fancy indexes, processing input files in various ways.

Like they'd even spawn arbitary processes for you (w/ CGI).