HowTo: Open page from Internet Explorer (Metro) app into desktop IE
The Windows 8/8.1 app version of Internet Explorer is also known as IE Metro because of the “Metro” codename (inspired by navigation signs in public transport] of the Modern UI design language promoted by Microsoft).
However that version isn’t the full Internet Explorer, in that it is unfortunately not supporting extensibility via plugins in the form of ActiveX controls as the classic (desktop version) of IE. It is only embedding the Flash player engine directly in its codebase, but not Microsoft’s own Rich Internet Application (RIA) rendering engine aka Silverlight, nor Unity or other VRML/X3D, QuickTime/QuickTimeVR etc. plugins.
Browser pages cannot detect the difference between running IE on the desktop or as an app, there is however a workarround for webpage authors or webadmins to force the app version of IE to show a prompt to the user that allows the opening of a page in the desktop version of Internet Explorer. There is also a way for System Administrators to set specific sites to open in the desktop version of IE without the user seeing such prompt.
At https://msdn.microsoft.com/en-us/library/ie/hh968248.aspx, Microsoft mentions:
As a web developer, you can enable the requiresActiveX feature switch either by using this HTTP header:
X-UA-Compatible: requiresActiveX=true
Or by using this meta element on each affected webpage:
<meta http-equiv="X-UA-Compatible" content="requiresActiveX=true"/>
I just added the meta tag inside the <head>…</head> block of the Amnesia of Who web version that uses Silverlight and here is how it shows in the IE Metro version (note that Silverlight IS installed in that Windows 8.1 machine, it’s just that it’s not available in that browser, that’s why the Silverlight installation prompt is also shown):
When the user presses the default button “Open on the desktop”, the OS switches to classic desktop mode and shows an Internet Explorer window with the Silverlight application starting fine (or if Silverlight is not installed it will prompt and allow the user to install it – note that Silverlight ActiveX control’s installation doesn’t need administrator permissions since that installation doesn’t affect other users, nor requires any elevated rights in the system to work).
I hope that Microsoft, apart from keeping on supporting this workarround, will do a clever move this time and embed Silverlight too (apart from the Flash engine that was in IE Metro) in the Spartan browser that it prepares as the Windows 10 default touch browser. And why not, provide some extensibility method for it, since HTML5 cannot become a huge, impossible to implement beast, that covers every future conceived functionality for the web.
George Birbilis @zoomicon 
Microsoft Edge browser (from Windows 10) doesn’t seem to support this mechanism unfortunately. However, there must be some other mechanism in that browser to show an “Open in Internet Explorer” button and to remember and automatically open a page in IE11 when one has visited it with Edge.
That is since if you go to this page:
http://catalog.update.microsoft.com/
with Microsoft Edge you see a special page that allows you to open it in Internet Explorer