BulletML is a markup language that describes the barrage of bullets in shooting games. - https://news.ycombinator.com/item?id=410898 - Dec 2008 (2 comments)
https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...
It was rrootage that sent me down a rabbit hole and eventually stumbled onto fal-works and firedancer after someone suggested their works had the same aesthetic appeal.
The libbulletml c++ library is still somewhat used in some niche vertical shmup games (mostly Japanese indie games). You can read a little about bulletml and the history of it all in the libbulletml readme file.
[0] https://en.wikipedia.org/wiki/ABA_Games
[1] https://github.com/abagames/rrootage
I saw “applet” and thought it was retro, then someone tried it in firefox 3 and I had to check the year.