Weird HTML syntax working on codepen but not locally

I want to modify this

https://codepen.io/YusukeNakaya/pen/XyOaBj

the html says

.night
  - for (i = 0; i < 20; i++)
    .shooting_star

I have never seen this syntax before, and if I use the css from that link in and the html in

but only the background renders, not the stars, but no errors are thrown,

Any help appreciated