1. some simple selectors can be used in specific contexts (like `html` for type inheritance, for instance). `flex-module` is a component name (albeit a very generic one only used for example purposes...) and `is-loading` is a "state class". I use simple prefixes to distinguish between selector types and `is` is one of them.

2. Breaking changes could be introduced through the Tailwind dependency tree. Some update could be necessary to patch an exploit, again breaking functionality. Updates become routinely necessary. Which is not the case for native code. Also, using new CSS features can be more taxing within the TW ecosystem than in with vanilla CSS. In a nutshell, it's about simplicity, maintenance and the perenniality of your code. On the other hand, you may be "lucky" and never experience something like the above. But I'd wager there's more chance that you will than not. At least, my own dev experience tells me this.

3. Well, I'd say it's a matter of scale... I see 60kb of CSS on Tailwind's homepage. If more than 80% is unused, that's something like 45kb of CSS. I think at this amount, you could see marginal improvements in rendering and loading. At least, more than with comething like 2-3kb of unused CSS. Hehe. But yeah, as I said, marginal. BUT! hehe, with a less monolithic approach to bundling CSS, as with links in components, for instance, there's a real gain to be had: the browser delays reading the files not being used in the current viewport. So, if you footer is not displayed in the first screem it's CSS won't be read, accelerating overall rendering. This, in my experience, makes a palpable difference.

Hope I'm being clear, english is not my first language. And thanks for the mental workout! Hehe.