Is it an issue if EPUBCheck starts reporting HTML-in-iframes?

[rdeltour]

As described by @JayPanoz in w3c/epub-specs#1238, there's a "hack" currently being used to allow non-XHTML HTML content in iframe elements.

The spec says that such HTML documents (e.g. type text/html) should have a manifest fallback. However, unfortunately this doesn't seem to be actually checked by EPUBCheck, as reported in w3c/epubcheck#985.

The problem here is that this technique may be somewhat widely used (would anyone have usage data?). For instance, InDesign seem to produce such books (under some circumstances). So this becomes a community issue: would all hell break lose if EPUBCheck started to (correctly) report EPUBs using this hack?

[rdeltour]

/cc @CircularKen

[JayPanoz]

[Attempt to reduce Romain’s workload a little bit]

Ken’s reply: https://twitter.com/CircularKen/status/1101414022786793472?s=20

Don’t have further details though – and I’m not sure if his content is non-XHTML HTML.