Yep, it worked! Using the style attribute inside the 'table' tag works perfectly... thanks, Raphael! I'm marking your post as an answer! :)
Guess I wouldn't mind if this were mentioned on the GitHub page. It's not exactly obvious that you are not allowed to use the CSS file for table borders (and possibly other things). (I know that the question wasn't specifically about the .css file, but I find that when you include border formatting for table elements in the .css file, it doesn't work either - moving to inline style does.) Cost me a couple of hours.