This is not correct. In instances where CSS doesn't affect the styling of the control, the browser can use native controls. As far as I know, every browser _except_ Firefox does this. For example, <select> elements in Chrome and Safari are native elements (albeit Win32 versions in the former on Windows).
This is not true. It's not just the styling of the control itself that makes it infeasible to just stick an NSButton inside the browser's NSView; it's about the way a control interacts with everything else on the page.
Source: I work on browser graphics.