In Edge, quicknav of b / shift+b should move to buttons.
If this is implemented similarly to other form controls, currently this may locate buttons in composite controls where the button itself is not represented in the text pattern. For example, file upload, or audio and video tags. These particular issues have been reported to Microsoft (MS bug 8370271, and MS bug 4379303).

This should be implemented as normal, with the expectation that the above Microsoft bugs are addressed.
P2 as this does annoy some people.