[Wpack] Factoring out some parts of the Web Bundle format

[Daniel Ehrenberg <[e-mail]>]

Hi WPACK WG,

As I mentioned in the message "Broadening the WPACK WG charter", I'm
very interested in Web Bundles for subresource loading, e.g.,
https://github.com/WICG/webpackage/blob/master/explainers/subresource-loading.md
. The Web Bundle format has a great architecture, in terms of having an
extensible set of sections, to permit various different usage profiles.
This WG could use take advantage of this architecture in order to define
a cleanly layered format. In particular, a few features are not used for
subresource loading:
- primary URL: not meaningful, as the bundle is not being navigated to
- manifest section: not used, similarly--the manifest should be linked
from the document which includes the subresources
- variants: not recommended, as content negotiation should be done when
deciding which bundle to include, to avoid wasteful downloads

I think we can factor each of these features out from the core Web
Bundles Internet-Draft and put them into a separate Internet-Draft which
indicates how to use bundles to represent whole pages, much like the
"signatures" section was recently removed from the Web Bundles draft.
For this, I would suggest:
- The primary URL should be located in a separate, optional section,
like the manifest section.
- The primary URL and manifest sections should be factored out into this
other draft
- For variants, the story is a bit trickier, since it is integrated into
the responses section. Honestly, I'm having trouble understanding the
motivation for Web Bundle's variants support, e.g., what use cases it is
meant to satisfy. Some kinds of negotiation (e.g., with Client Hints or
UA) extends outside of the variants model, if I understand correctly. Is
there something else we can do to future-proof the binary format for
this support, rather than including this particular format in the core
RFC?

There are other additional sections which may be useful for some
applications. For example, within build tooling, a section could be
added to represent the dependency graph among subresources. So, Web
Bundles with this modified factoring would represent an excellent basis
for a variety of uses. 

What do you think?

Thanks,
Dan