border-collapse did not show up in my list of styles regarding borders. Why? Because it is there specifically for tables.

table{
border:solid red 2px;
border-collapse:collapse;
}
This is column 1 This is column 2 This is column 3 This is column 4
Here's data Here's data Here's data Here's data
Here's data Here's data Here's data

Now the gap is gone! The opposite effect would be border-collapse:separate;.

And *whoops* the blue border of <tr> suddenly shows up. We didn't want that now, did we.