I have a different take on this issue: I don't think polyfills are really
relevant for this discussion.
IMO, the kind of "publication mode" that we're talking about is very
different from things that are usually polyfilled on the Web, since it's
essentially a set of affordances and features.
We could attempt to boil this down into individual features, that could
then be polyfilled, but IMO that's the wrong approach:
- I don't think that we should polyfill features on individual resources
of a publication
- the publication shouldn't be in charge of controlling some of these
affordances (for instance, user settings should be consistent between
publications), even temporarily
- instead, providing a Web App that is only accessible from the entry
page (entirely optional) could be a fallback
- this could of course be a browser extension as well, which would
detect the manifest and trigger this publication mode accordingly