Basic (Grade C)
Browsers in this category are known (listed below) and identified via a feature test suite and a blacklist in the startup module.
In conversations, a number of voices were referring to “navigable”, “readable”. These attributes might be fine on a Readers experience side, they don't mean enough on a contributors side. And more importantly, such attributes are nowhere in there currently.
A second problematic ambiguity comes from the access or infrastructure perspective.
All three groups might have access, but ones that are falling out of access for reasons like old TLS support, are automatically in unknown. So Unknown is a mix of either no access or JS/no JS or CSS/flawed CSS browsers.
In my opinion clarifying the language would enable product teams to make better choices about the roads needed to be taken and also help in future decisions on changes like T248061