The problem is that the border I made for the thead
isn't following the rules that I made for it - the position, which is moved down, and a little to the left. How can I move the border so it "sticks" to the thead?
Try to avoid using positioning in a table. Positioning is intended for block elements, not tabular data as such. I don't think it is helping you achieve what you want in this instance.
Is this closer to what you want?
http://jsfiddle.net/GRf9v/189/
This should not be a table. See updated answer: http://jsfiddle.net/GRf9v/190/
Hopefully that will help you get going.