This is probably (well) known issues:

According to what CSS 2.1 says about the

empty-cells:hide

property, then, a table cell should be considered to be empty even if it contains line breaks. An UAs are required to treat all representations of the line-break the same. And so, a child

<br>

element should not cause the cell to be seen as non-empty. And this is also how Web browsers behave. However, Prince (version 9) sees such cells as non-empty (and therefore fails to hide the borders for these cells.).

Btw - and this is perhaps related, and I probably just forgot this bug:

br{border:1em red solid}

causes border to be be displayed. This is at least against Web browser behavior.