> Then you wouldn't have to remove default styling for it to "work out of the box"

I didn't. I removed the CSS styling provided in the example to test that it works perfectly without it.

As OP said in his post description "Base UI doesn’t bundle any styles"

> Barely visible thin light gray lines where the input is supposed to be are endemic

That's a very different complaint, and if as you say "endemic", contradicts your original one that in general they don't render at all.

I imagine that the CSS in the examples are intentionally lightweight exactly because they're unstyled components. It isn't a design system, so is being as unopinionated as possible while demonstrating how to style them.

I don't use the "T" word lightly, but it really does look like you're intentionally being one. (That or poorly informed – you chose which side of Hanlon's razor you sit.)