I'm currently working on a dashboard-type SPA, with no build tools. Just a single file of HTML, a single file of CSS, a bunch of plain JS files with an import map, and a single dependency on d3 (also imported as a module). In total about 10Kloc.

I have to say, it's been a few years since I last did front-end dev work, and I was really pleasantly surprised by how much better CSS and Javascript became. All in all, it's been a really positive experience.