> If you want to build a cross-platform UI product that doesn't require installation and has Emacs-like levels of extensibility, JS is the end of the line in terms of language selection. There are no other candidates.

calibre (with python+Qt) begs to differ. And I believe VLC runs on every current platforms. There's also Code::Blocks and Blender. Cross-platform UI is not rocket-science.

All great examples, but ones that do require installation. Their website is just where you download the product, I'm talking about making the website the product.