According to the tbl(7) manual, if a data cell contains only the

According to the tbl(7) manual, if a data cell contains only the
two character sequence "\_" or "\=", a single or double horizontal
line is supposed to be drawn inside the cell, not joining its
neighbours.

I am not aware of any way to do that with HTML and/or CSS.
Still, it seems closer to the intent of the document author to draw
a horizontal line with <hr/>, even though that line will join the
neighbour cells, rather than printing a literal '_' or '=' character.

Formatting tweak inspired by a related question from
Ted Bullock <tbullock at comlore dot com>.