Because that third-party code adds its own CSS.

Because my HTML and the third-party HTML live in the same document. Thus, my CSS could target their HTML, and their CSS could target my HTML. There are no modules and therefore no module boundaries.

Because my CSS and the third-party CSS share a global namespace. Thus, their CSS selectors might overlap with my CSS selectors. Hopefully nobody called anything ".container" or ".wrapper" or ".button" since there's no namespacing, right?

Keep in mind these components might be arbitrarily nestable. Maybe my elements can be a child of theirs? Maybe theirs can be a child of mine? Can I opt out of the cascade? Should I?

Maybe we could all use the Shadow DOM? I don't really have control over what third-parties do, but let's say I dig into their implementation (what fun) and choose only ones that use the Shadow DOM. Now I've massively bumped up the complexity and ensured my document can't render without JavaScript; lots of user-agents go right out the window and see nothing.

Let's reduce it to the simplest possible case: you and I are working on the same website, but I'm responsible for the header and you're responsible for the footer. We even have our own separate .css files. Yay, no merge conflicts.

Can you and I safely write our selectors without coordinating with each other?

(If you think the answer is yes... you're wrong.)

Now add a few dozen more team members and several third-parties who we in fact CANNOT coordinate with.

Everywhere you turn there are tradeoffs and half-solutions. Turns out "Just Do X" is not very helpful. Thus, I don't blame people at all for developing their own solutions to scaling CSS - like OP's framework.

There are still no affordances for many things, so people have to invent their own solutions.

That misses the point. They're identifiers scoped to the entire document - the same way global variables and global functions are identifiers scoped to an entire program.

One of my earliest webdev jobs was at a company whose product was a widget you could add to your site by adding our `