This had indeed no effect. But the difference between the FRC and Ultima Diagram is that the latter had the coordinates displayed. So I added a firstRank=1 to the FRC Diagram definition. And now it does show up.
It seems the absence of the rim cells causes the height of the table cells to collapse to a height of 0px.
[Edit] The inspector says the <td> cells corresponding to board squares still have a height: 0px. But the left rim cell has
This had indeed no effect. But the difference between the FRC and Ultima Diagram is that the latter had the coordinates displayed. So I added a firstRank=1 to the FRC Diagram definition. And now it does show up.
It seems the absence of the rim cells causes the height of the table cells to collapse to a height of 0px.
[Edit] The inspector says the <td> cells corresponding to board squares still have a height: 0px. But the left rim cell has
height: 50px; width: 25px; height: min(10vw, 11vh, 50px); width: min(5vw, 5vh, 25px)
and is apparently forcing the rows open.