I'm a fan of Formatting & Linting, here are the tools I normally use.
StandardRB for Ruby projects. Built on Robocop, but less strict and no configuration required.
SlimLint for Slim templates.
StyleLint for CSS.
These can all be integrated into Neovim, although I haven't setup SlimLint or StyleLint with the LSP framework yet, actually not sure they have LSPs.
I use Overcommit to manage local Git hooks to make sure these, and tests, run before committing and pushing to GitHub. I will also setup GitHub actions to run them again on the server.