Can I copy some random HTML+CSS snippet from the internet and be sure that it'll look exactly the same in my project and that no existing CSS is going to overwrite it?

I'm never gonna argue learning proper CSS wouldn't be better, but Tailwind is by far the path of least resistance for someone that has no interest in writing frontend for a living. It's like putting legos together, it requires very little thought to get from nothing to a decently looking website.