I’m posting this to hopefully save someone a little time when they run into the same issue I did.
I was trying to add an RSS Viewer web part to a page in SharePoint Online (E3 plan) recently and got the following error:
A Web Part or Web Form Control on this Page cannot be displayed or imported. The type is not registered as safe.
I was little confused since the RSS Viewer web part is clearly available in the web part gallery, but then I found a Microsoft support article confirming this web part “is not available in Office 365″ and “is scheduled to be resolved in a future version of Office 365″ (not sure if that means it’ll get fixed or get removed… guess we’ll find out).