[Push API] How to detect acceptable Content-Encoding for each UA

Hi,

I would like to make a comment on a concern about using Push API with Web
Push Encryption.

Web Push Encryption spec[1] says an application server must indicate
Content-Encoding so that a UA can correctly process received push messages.
Currently there are browsers which supports "aesgcm128" Content-Encoding
(e.g. Chrome 49, Firefox 44+).

On the other hand, it could be renamed according to encryption spec update
(e.g. changing from "aesgcm128" to "aesgcm"[2][3]). It might suggest each
UA might support different types or versions of Content-Encoding in the
future.

If I understand correctly, there might be no manner to notify the
application server of Content-Encoding which the UA could accept, without
checking UA's name or version. IMHO, if Push API would provide any method
or attribute to indicate acceptable Content-Encoding, it would be helpful
for the application server. WDYT?

Best regards,
Tomoyuki Shimizu

[1] https://tools.ietf.org/html/draft-ietf-webpush-encryption-01
[2] https://lists.w3.org/Archives/Public/ietf-http-wg/2016JanMar/0180.html
[3] https://github.com/httpwg/http-extensions/pull/137