[css-regions] IFrame as flow content and fallback content

Hi all,

While I didn’t see (in the current draft hosted on w3.org) a reference to the use of IFRAME, OBJECT, EMBED, LINK or @flow as flow content for a region, I remember clearly this was discussed in the past. 

That feature seems really attractive to me, but I’ve come to an issue: while the resource is still loading or if the resource fails to load, how can we define a “fallback” content that will be sent to the flow instead? Can we rely on some sort of fallback chain? 

I know the OBJECT tag already allow some sort of fallback content but doesn’t allow granular control about that fallback (what if the OBJECT is still loading, failled to load, has content which wasn’t recognized or has content that is not accepted as a flow content).

This can be an issue for real-world usage of the feature, I think. If those use-cases are targeted, that issue should be fixed somehow.

Best regards,
François