What if a user needs it ready-to-use? Also imagine you’ve created some styleless form which I need to style according to my Bulma or Bootstrap stylesheet. What if the structure is different than these two are assuming?
I'm not suggesting the layout shouldn't be included at all. There's nothing wrong with including a default for the component - as a stylesheet in "examples" or something. The point is that layout shouldn't be a part of the component itself. The beauty of CSS is that it makes doing that absolutely trivial.