This is an attempt to clarify the support definition of “Modern”, “Basic” and “Unknown” in front-end technology terms outside of the technical infrastructure access:
Modern: Everything works as fleshed out in product design (and features take advantage of browser's full capabilities)
Basic: Some features might or might not work, but basic functionality is given, and enhanced functionality degrades gracefully. Examples are basic editing or search.
Unknown: Some browsers might get full features, some not, some even break badly.
Modern: Full layout & design features
Basic: Full layout features, we use same CSS, as it might otherwise negatively affect degraded JS functionality. We go through hoops and use Basic as CSS level for our designs incl Modern.
Unknown: Not cared about. Given we're only using semantic HTML basic readability is ensured (only exceptions are browser bugs)