I am not even sure if it is a bug in WebKit or just Yahoo acting idiotic, but let me try. Both Camino and WebKit are not supported browsers fro Yahoo Mail Beta, but with Camino it works and WebKit it throes an error I will attach.
URL that ignore browsers is http://us.f395.mail.yahoo.com/dc/launch?sysreq=ignore
As I said I don't know if it is WebKit Bug.
Created attachment 13606 [details]
Yahoo Mail Beta Error
Confirmed with a local debug build of WebKit r20141 with Safari 2.0.4 (419.3) on Mac OS X 10.4.8 (8L127).
NOTE: To reproduce this, you must log into the "normal" Yahoo! Mail, find the server that you're using (e.g., us.f377.mail.yahoo.com), then change the hostname in the URL in this bug to match your server. If you don't do this, then you still get the web page telling you that your browser is not supported.
I'm not able to reproduce the issue now. Yahoo developers fixed this problem and Yahoo Mail is officially supported in Safari 3 for Mac now: http://www.ymailblog.com/blog/2008/03/some-all-new-features-in-the-all-new-yahoo-mail/.