Arena was the testbed browser for HTML 3.0 created by the W3C
(www.w3.org).  It is now being maintained and improved by Yggdrasil
Computing (www.yggdrasil.com).

The original description from the W3C: 

Arena's primary purpose is to be a testbed for new features of the web. In
the past, we have implemented HTML3 features including tables and math.
Currently, style sheets are in focus along with new graphics formats such
as PNG. Also, Arena is a testing tool for the W3C Reference Library. The
library provides a common platform which will ensure future compatibility
on the web. If you are a browser vendor, it will save you time.