The collection returned by YUI's getElementsByClassName returns "[object HTMLAnchorElement]" instead of the expected value. This is a regression from Safari 2.0.4 and Firefox. However, as mitz pointed out on irc, when accessing the array item individually (getElementsByClassName('foo')[0]), the correct value is returned. The testcase I will upload shortly shows this more clearly. I've cut down as much of the unnecessary bits from the yahoo files as possible.
Created attachment 10384 [details] testcase
This is apparently a regression from r14463 (patch for bug 8964, autogenerate HTMLAnchorElement).
I think this HTMLAnchorElement regression exposed a bug in Array.prototype.toString() that was introduced when bug 3991 was fixed (r9842): according to the spec (15.4.4.5), unlike toLocaleString(), join() (and thus toString()) should not use the array elements' toString(), but rather apply the ToString operator to them.
Filed in radar as 4808136
It appears that the fix for bug 11524 in r17610 addressed this regression.