W3 specifies that only four CSS rules are allowed for table columns (with the <col> element) - border, background, width and visibility.
Does anyone know the reasons behind this decision? If you can have borders and backgrounds, why not fonts and colours?
Possibly because each row in the table does not necessarily have to display a cell for your column (e.g. because of a colspan). Which column should that cell inherit it's style from? Just a guess.
Just a wild stab in the dark based upon my limited understanding:
I think styling via column related elements is restricted because although <col> and <colgroup> represent columns of cells, it does not actually contain them (they're actually contained by the <tr>s). With this comes issues of precedence and specificity and cascading (since cascading can only be done between contained/ container elements) - when conflicting style rules from the <tr> and <col> (which would be the same level in a multiple inheritance hierarchy) occur - which should the cell actually use?
As to why that particular handful of style attributes is permitted though: no idea.
One word: ambiguity. The cells have to be children of rows; it wouldn't be a table otherwise. But there is no column to descend from. Using colspan means that one cell could be in two columns. Rather than trying to come up with some confusing way out, why not just let the developer place a class on every nth cell?
If you look closely at the spec to which you link, you will see attempts at ambiguity resolution. The width property specifies a minimum; the background takes a backseat to the row and cell; and border references a "conflict resolution algorithm". The only reason there is even an algorithm for border is because it is reasonably understood who should "win" (see the algorithm for details). But could you imagine trying to figure out which color or font should "win"?
Ian Hixie explains in detail at http://ln.hixie.ch/?count=1&start=1070385285