Testcase in
WYSIWYGUnknownTags.
SP
No, actually Kupu strips unknown tags. The plugin is just a vehicle for Kupu. I'm not about to escape unknown tags!
CC
Must admit that I cannot come up with a specific case in our setup where this would be a problem either. Should be something in the lines of a javascript menu with an internal format for menuitems marked up in its own tags or similar.
Hate leaving out the <nobr> though, some folks use that quite a bit (mostly inside tables). I presume they are able solve this by some clever CSS'ing instead (nowrap property for td or smth).
SP