Excel Online - Tell us what forces desktop Excel so we can write xlsx files that work in Online - or make online=desktop functionality
Tell us the gotchas that force Excel Online to desktop so we can write spreadsheets that don't force desktop. Going back and forth between 2 different versions of the "same" product is so kludgy.
Or even better when it opens in online version with this error message, identify where the incompatability is so that user can stop using it. (my spreadsheets are very basic yet I get the error message)