When I migrated one of my apps to Windows 8.1, I found that some code I had attached to the body.onload event wasn’t getting called. Windows.UI.WebUI.WebUIApplication.onactivated is the last thing that gets fired from the app host on startup (barring WinJS.Application.onready which is just queued after that but not related to a DOM event).
It turns out that there was a breaking change between Windows 8/WinJS 1.0 and Windows 8.1/WinJS2.0, which changed the exact sequence of the Windows and WinJS events (a sequence which was never specified nor guaranteed in the first place).
Note first that document.body.onload generally won’t work—one should use window.onload instead.
In WinJS 1.0, window.onload fires after WinJS.Application.onactivated. In WinJS 2.0 it fires before all the WinJS.Application events (including loaded, activated, and ready). My Win8 code was adding a window.onload handler within WinJS.Application.onactivatedt seeing it any longer. So I simply needed to move the addEventListener call earlier in the code and everything worked, but it took me a while to figure it out the cause.