[css3-ui] tooltip for ellipsed text (Issue 49)

https://wiki.csswg.org/spec/css3-ui#issue49

When text is being ellipsed, and the user hovers over it, safari shows the full text in a tooltip, while other browsers don’t.

http://jsbin.com/quhisipiba/1/watch?output

I think this could be worth standardising, as authors sometimes create this tooltip via js, clashing with the native tooltip of browsers that do provide it.

I do not have a strong preference about whether browsers should natively show the tooltip or not (although it does sound like a nice thing to do), but this is probably worth being consistent about.

 - Florian