[css-view-transitions-2] Handle design changes between pages in MPA transitions

Unlike SPA, regular navigations may be from version 1 of a site, and version two. In some cases, version 1 and 2 may be incompatible in terms of view transitions - such as a major design change.

We should have some mechanism to handle this.

One way would be to have a version ident as part of the opt-in:

@cross-document-transition {
  state: allow;
  version: 123;
}

And if the versions don't match on either side, the transition doesn't happen (as if the 'new' state didn't opt-in).

If we go with the pattern above, we need to figure out what type the version should be, as a number seems a bit restrictive.

Although, is a version too blunt an instrument here? Developers may be in a situation where going from version-1 index to version-2 profile is fine, but going from version-1 index to version-2 article isn't fine.

No files, tests, or implementation entry points are identified. Start by reading the issue and its linked opt-in proposal, then resolve how cross-document transition versions should be represented and whether compatibility needs more than a single version. Done means the behavior and scope are agreed for the specification.