> In any event, this raises a bigger question. What if a site is actually broken in Firefox, but it's not Firefox's fault? Should Firefox then be changed to non-conforming behaviour, aping any Chrome bugs?

I can remember one case of such a thing happening before: css box-sizing.

Before the attribute existed, the spec said content-box was correct, and all browsers except IE implemented it that way. IE implemented border-box, going against spec, but it turned out to make so much more sense it was eventually added to CSS, and nowadays is the recommended way to go (but still not the default value).