I haven't used Slim before, but looking at the examples shown on the repo's README, it immediately brought to mind the thought experiment: 'what if Haml but Yaml?'

n.b., I've been building Rails apps since 2007. I was a diehard Haml user up until about 2021 when I made an all-in bet on Tailwind and had to switch back to ERB because Haml with Tailwind felt too obtuse.