Dear Alain.
I think we should update the actual content based on the actual Trac default wiki syntax, or if there is another way to make it ...
I have tried to add some HTML code. Take a look at this example:
https://projects.centos.org/trac/artwork/wiki/DistroDesign/Wallpaper
As <style> definitions are not accepted I have to use dirty tricks to adjust table borders. I wonder if this really makes sense.
Maybe we should just stick with the capabilities of the Trac wiki (which definitly means col/rowspanning is impossible) and re-work the content.
Best Regards Marcus