i knew i'd seen this somewhere before... it's juggling lab, i'm pretty sure. i remember playing with some applet of it embedded in a website in like the 00s. good times
i'm glad!! also, an update: o hell yeah they have a server-side gif renderer too… you just need to learn uh. Juggling Notation. to be able to create a better, stronger final boss
as it turns out, the siteswap notation for the juggling pattern is just 3, but to get the final boss, you need to specify hand movement
in the documentation for hand movement formatting, the movements for Mills' Mess (the juggling pattern in the animation) are given as an example
The standard style for Mills Mess is "(-30)(2.5).(30)(-2.5).(-30)(0)." if you care to puzzle that one out.
so to get the final boss from jugglinglab.org you'd pass the parameters pattern=3;hands=(-30)(2.5).(30)(-2.5).(-30)(0).;colors=mixed

you could get more balls with something like pattern=5;hands=(-30)(2.5).(30)(-2.5).(-30)(2.5).(30)(-2.5).(-30)(0).;colors=mixed

or longer arms with something like pattern=3;hands=(-300)(2.5).(300)(-2.5).(300)(0).;colors=mixed

the original gif probably came from the Wikipedia article on Mills' Mess
parameters are as follows:
pattern=<3p|3p|3p>;body=<(315,-100,20,100).|(0,0,0,100).|(45,100,20,100).>;colors=mixed;camangle=(0,110)

