Well, I deleted the width="nn" clauses in the cells, and now everything is ok (no vertical borders). But I needed these clauses
, because inside some cells, I repeated a thin .bmp band to form a wider horizontal band.
Otherwise, I'd've just splashed the whole damn picture ! But, I was trying to reduce image size. Wasn't that thoughtful of me ?
Why .bmp and not .jpg ? Because over the band there are superimposed some other drawings, and the band has a pure color, which .jpg turns into small, irregular, ugly variations of the same color. And also, one drawing is larger than the band itself, both above and below the band.