Specifically, the highest priority border is double, followed by solid, dashed, dotted, ridge, outset, groove, and lastly, inset. A cell border has precedence over a row border, which trumps a row group, followed by a column, column group, and lastly, the table itself. There are a few nits and caveats to this whole process, of course. Since different border styles will meet at element corners at some point, the browser must consistently render the mitering of two border styles as needed. Given the effort by the CSS2 designers to resolve the border conflict itself, exhaustion probably forced them to defer border corner resolution until at least the CSS3 specification.
Collapsed borders exist between elements.
If there are an odd number of pixels to be distributed around a border, the browser must have a consistent way of doing so. Again, CSS2 does not say how this might be done, so browser designers can go wild here. As we pointed out last week, borders, both collapsed and separated, have their uses. The good author will learn them both and use them as needed. The Definitive Guide.