Consider adding a way to color table cells

[shaunix]

It's often useful to use background colors on table cells to help information stand out. For example, a feature comparison table can use a green background on "yes" cells. I don't want to go down the route of having authors specify exact color values, but yelp has a collection of named colors it could easily use (http://yelp.io/xsl/color).

This could be done with an attribute, or it could be done with style hints. For example:

<td style="green">yes</td>

Even if we do style hints, where tools are free to ignore them, there's precedent for having recommended hints in the spec. I'm leaning toward having some recommendation around this in the spec, because I think it's very generally useful.