WebGL is a very high level API because OpenGL itself is a very high level API in this context. People have been coding directly on top of OpenGL since it was first created, including 4k demos. A demoscene demo will itself implement whatever custom "graphics engine" is needed (for a 4k, essentially nothing is needed). Demos don’t commonly use third-party dependencies because the entire idea is to show off your (group’s) programming and art savvy.
They are good ways to distribute programs for customers but that’s it.