What about clients' obligations with unknown elements? My expectation would be that clients preserve them. Say that my server has an entry
<entry> <title>Tintin's Dog Snowy Is White</title> <content>Herewith, some notes on Tintin's dog.</content> <tintin:snowycolor>white</tintin:snowycolor> </entry>
and tintin:snowycolor is unknown to my client. If my client wants to change the title to "On the Pigment of Tintin's Dog", it will GET the entry, make a change, and POST it. I would not expect the data in the snowycolor extension to get lost.