experiment with trailing effects

I'll probably have to resort to javascript and using canvas effects to
draw a path or a trailing effect.
two-beat weave

it's a little silly that in order to get the overlap correct I had to
make two separate animations, but no one said CSS would be completely
serious.

I'm already starting to notice the limits of css animations on my
laptop. Maybe I can achieve a similar effect with a less complicated
animation timing function.
refactor two-beat-weave