Hello,
I've noticed this too and it is annoying that the default table items are styled with a thin border.
We would appreciate that the table layouts are by default out of any borders (especially for people pasting signatures that use tables).
I also played a bit with element inspector to discover from where this unwanted styling was coming from and find the same as you.
That being said I've found that you can change the table style in the editor for a specific table here:
Still, IMHO the "No borders" should be the default, at least for signatures editing...
Kind regards.
Sébastien Riccio
System & Network Admin
https://swisscenter.com