6.1. Netscape Navigator 4.x

Very old.
Relatively Unmaintained.
Buggy and Incomplete CSS support.
Incomplete or completely lacking support for newer standards.
As a result, it tends to break under many pages.
Very small (and descreasing) use percentage, but can still be found in god-forsaken (mainly old) installations.
My advice: ignore completely. Supporting it is not worth the trouble. Your kilometrage may vary, however. (depending on the circumstances)