Thanks for taking time to reply.
In fact, I'm copying and pasting text only within the ArcGIS Viewer publishing framework. I use one AGO map app as a template for Details page, tags, etc. for all my maps. I've spent a good deal of time and effort carefully formatting the Details page for this template. Shame to dump all that by pasting the text into Notepad.
So the problem originates mostly inside AGO itself. I noticed while troubleshooting that the editing feature in the ArcGIS.com Viewer does not allow us to choose "no background". So when I clicked "white" to try to remove the dark background that was produced in the Compare Viewer, I was stuck with an explicitly white background. Can't get rid of the background.
OK... Starting over with Notepad. So much for using an AGO map template...