Shaders!
Remember the early Windows screen savers? Theirs drew polygons by default, but I think the line version looks better. Implemented without branching or trig functions. Has an intentionally low framerate to get the retro feel.
Also on ShaderToy
Wanted to give ray marching a try. Turned out a bit Escher-esque.
Also on ShaderToy
Thanks to Rabbid76 for sharing their code for moving a Shadertoy shader to a stand-alone page.
MIT