How it will be with I web on a new system Capitano?

I'm sorry but iWeb was dropped with the release of Mountain Lion.

IWeb was discontinued buy Apple over 2 years ago and is way behind other apps in terms of features. This would be a good time to consider migrating to a modern, website development application.

Do a search in these forums for "iWeb alternative" or iWeb replacement" to get many conversations on the options. One that many like is EverWeb which is very much like iWeb in its user interface and ease of operation. It has a demo version you can try.

Nobody knows, because this OS has not yet been released.

There is a chance now that it won't work any longer and as iWeb was dropped and is not supported by Apple any longer and this has been the case for over 2 years now, then this might be a good time to move on and leave iWeb behind.

There is much better software out there now that can do more.