The MS Edge browser from Windows 10 is NOT supported for use with 12.1 Web Viewer. There are no plans at this time to alter 12.1 Web Viewer so that you could use Edge to access the application.
MS Edge does not support the use of ActiveX Controls which of course impacts emailing and printing from the Web Viewer. Edge also does not support Modeless Dialogues. Modeless Dialogues are used throughout 12.1 Web Viewer. Much of the application would have to be rewritten in order to be used from the Microsoft Edge browser.
As CAOM Web Viewer does not interact directly with the client machine operating system (for the most part), users may employ MS Windows 10 when accessing the Web Viewer, providing they use a supported browser like MS IE 11 or Mozilla Firefox.