[css-ui] text-overflow: ellipsis and user agent tooltips

I noticed today that Safari 6 and Webkit Nightly (but not Chrome) show a
native browser tooltip when a user hovers over any text that has been
truncated via the `text-overflow: ellipsis` declaration. The tooltip that
appears is the same as the tooltip that appears for elements with a `title`
attribute.

I looked at the current draft of the spec [1] and didn't see any mention of
what user agents should or should not do in this situation.

I don't know if this has been discussed here before, but perhaps it's worth
being explicit about the desired behavior. Currently, if you display your
own JavaScript tooltip in these cases, your Safari viewers will see two
tooltips.

[1] http://dev.w3.org/csswg/css-ui/#text-overflow