What SDK or type of development language for panels in Bridge CC

Hi,

I want to write my own filter panel in Bridge CC and I'm a little bit confused about the technology to choose.

Do I have to write javascript code and build the UI with HTML5/CSS?

Or do I have to choose Flash/ActionScript or something like Flex?

Then I read something about the "Adobe Extension Builder" but the homepage behind seems to be a little outdated (the 'try'-download isn't available anymore).

I'm looking for an entry point to start with the development (and I hope, this forum is the right address for such a question).

Thanks

Konrad