minimal ua styles for tabs

[bkardell]

We currently are inferring things about layout for tabs, but then do nothing at all beyond that. From my own experience with them just now and from the demo, it would appear to me that we should at least provide a default margin-right or something to separate the tabs, else the headings text just run together (you can see this in the demo page). It also seems that something minimal to indicate the selected tab is actually necessary as well. Could be a background color or simple bottom shadow or something.

I have my own thoughts on how to best to try this, but @jonathantneal often is pretty good with matching expectations and I'm curious to hear how he would add UA styles for this so it doesn't screw up some other expectation - he has pretty good intuition/ideas here I think.

/ @davatron5000