the javascript ecosystem is the only one people use because just about every other ecosystem treats user-facing web like garbage which is why people keep building SPAs or having a separate node server that talks to an API server
you have a "web framework" and it already has it's own "build step" so why don't you have literally anywhere for me to plug postcss and/or typescript into. no, stackoverflow commenter. loading CSS via JS at runtime is not a solution for tree-shaking per page CSS
