What is the "open web" ?

All,

A variety of views have been expressed (in the "other place" where these
discussions continue) on the definition of "open web" and so I have a
question about where, exactly, the consensus ends.

I think we all agree that W3C recommendations must be implementable,
royalty-free, in open source software. This much is clear in our policies.
Some would extend this to "Free Open Source Software".

It's also been said that "open web" refers to an ambition that the entire
software stack on which the web platform rests be implementable in FOSS
software. That is, not just that W3C specifications should be implementable
in FOSS, but that the underlying capabilities the web platform exposes
should meet the same requirement.

This is not true today (see below for examples), but the argument is that
just because some existing things are "broken" is no reason to introduce
new things that are also "broken". People are working on fixing this for
the existing things.

Examples include Geolocation and WebGL. Whilst it is possible to implement
both of these in open source software, you basically need proprietary
hardware (and the proprietary software drivers to go with it) to offer a
performant capability to applications (GPS and a graphics card,
respectively). You could also include some video codecs, though here the
issue is just the royalty-free part rather than the open source part.

These are examples where there exist proprietary platform capabilities
across multiple platforms which the Web Platform exposes in a consistent
way. That is, the capabilities are sufficiently similar, (even if they
don't have identical platform APIs), and sufficiently widely supported,
(even if they are not on all platforms), that User Agents can (1) take care
of abstracting away the platform differences and exposing a common API to
web applications and (2) provide a less performant software fall-back for
platforms where the capability is not available.

Is there consensus that such capabilities are part of the "open web" ?

What is the bar for "sufficiently widely supported" ? Presumably if a
capability was available only on one platform then a web API for it would
not gain much support. At the other end, if there was only a single example
of a platform which did not support a given capability, would that be ok ?

What is the requirement for the "fall back" software implementation ?
Presumably there should be some applications that can work acceptably using
the fall-back. Is it required that all applications work in some fashion,
or is it acceptable that some applications do not work at all ? I imagine,
for example, that a navigation app probably can't work at all without
GPS-equivalent geo-location.

...Mark