Thanks very much for your reply, Tobias. I managed to get it all to work, although with some tinkering.
For one thing, to make the table-19 work (to remove the borders) I had to add “!important” to the border style.
I also needed to put some back that you suggested I remove. In particular, I found the font sizes became huge and the cell padding was also huge. I had been declaring font sizes in ems, so I needed to put that default back in, rather than go through all tables and change the font-sizes (I used 11px rather than 8pt). For much the same reason, I needed to put a default for cell padding back in.
I thought I’d mention these, since others might be having the same problems.
Many thanks for your help!