If the layout or data of an individual cell in a tbl(7) contains

only "_", "-", or "=", requesting a horizontal line to be drawn
across the middle of the cell, print <hr/> in that cell in HTML
output.

That is arguably slightly ugly because HTML 5 regards <hr/> as
semantic markup, meaning "thematic break".  If somebody knowns
a better way to render a horizontal line across the middle of a
table cell with pure HTML and CSS, and without implying a specific
meaning, please tell me.

Missing feature reported by <Oliver dot Corff at email dot de>.