I use Pano2VR and Object2VR, each of which output HTML/CSS & use a couple of custom JS elements to display 360° content. Initially I got stuck following the Javascript setup instructions here, but actually it was easy to do.
Once I figured out how to do it properly (by setting up each scene with a separate title, and call it via a custom URL) I was able to make certain hotspots inside the HTML widget trigger those external URLs. It just called the full URL including the appropriate #SCENENAME with a _parent target. Actually very simple.
You can view the final piece here Object VR demo
Only thing I wasn’t able to do in time for the completion deadline was work out a way of triggering a fullscreen action, which I’d like to have done. That uses a JS command in Object2VR, and from memory I couldn’t seem to get that to affect anything outside the HTML widget. I should try and revisit it sometime.