just wanted to know, as kontalk uses XMPP with extensions, if those extensions are standardized by the XMPP Standards foundation (XSF)
Most, but not all of them. You can find a list of used XEPs and custom extensions here: https://github.com/kontalk/specs/blob/master/README.md
Feel free to ask about a specific extension. Summary for compatibility with other clients: Exchange of (unencrypted) text messages or attachments should works fine. Beyond that, there could be problems: e.g. the current group chat protocol used by Kontalk is totally custom, i.e. non-Kontalk users won’t be able to join/participate in a group.
could these custom extensions be standarized? and, why kontalk, if it uses XMPP, doesn’t have XMPP features?
Some of them were standardized right after we developed another one for ourselves, so we will gradually port the code to the right XEPs. Other custom extensions will in time be proposed as XEPs as well.
I’m not sure what you mean.
I meant, Kontalk uses XMPP, right? and for example, in XMPP you can send videos, but kontalk can’t. Why? An extension has to be implemented?
Yes. XMPP has a solid core with a lot of optional extensions, see https://xmpp.org/extensions/index.html for the full list of available extensions.
To support a specific feature all involved entities (server and/or client(s)) must support the XEP extension for this feature. Implementing the support for a client requires some time so we can’t simply support everything.
And sending/receiving videos is possible with Kontalk, is it not working for you?
Are you sure about this? Going to Attach>Video returns a ‘Not implemented yet’ message.
oh, correct. It seems to be disabled in the Android client.