So, over in UXR, we are coders who want to be designers 
Our tool of choice is to build addons using the Addon-SDK (https://addons.mozilla.org/en-US/developers/builder).
goods:
*. it’s already html / css and uses the real stack (any work done at that layer is reusable)
*. This works fine for working with desktop, but is terrible for mobile
*. things are always ‘real’
*. easy to install / test / instrument on real installs.
bads:
*. doesn’t work on mobile (yet)
*. hard to affect the ‘deep chrome’ stuff
Takeaway: If you can get it to html5/css, we can make it ‘real’.
(some of the experiments we have done: https://github.com/gregglind/iconcontextmenus , https://github.com/gregglind/puggle )