Also, to more appropriately answer your question, there exists a notion for separating "document" and "UI widget" fonts. Documents often have content to be read at long lengths. UI widgets don't have the "long length" constraint in their design context. As one example, the system font may be lighter in weight to achieve an elegant looking aesthetic. However, while the lighter weight font is acceptable for the UI widgets, it isn't nearly as acceptable for readability of document text. It's a nice to be able to have that design/usability context distinction. The default context for the web is that of the document, not the UI, the default CSS sans serif font should be the "document font".