Proposal: adopt a "test required" policy for spec changes

The W3 leadership has been emphasizing the importance of ensuring
interoperability through Web Platform Tests. Some groups have adopted a
policy of requiring corresponding web-platform-tests pull requests for
before landing normative spec changes. Since interoperability is part of
getting a spec to become a Recommendation this makes sense especially for
specs that are in or entering CR. Should we adopt such a policy?

The Web Performance group adopted the following:
[[
ALL normative spec changes are generally expected to have a
corresponding pull request in web-platforms-tests, either in the form of
new tests or modifications to existing tests, or must include the
rationale for why test updates are not required for the proposed update.
[...]
]]
https://github.com/w3c/web-per <goog_1391446905>
formance/blob/gh-pages/CONTRIB <goog_1391446905>UTING.md

... and the CSS Working Group adopted one last week:
[[
For normative changes for any specification in CR or later as well as the
pre-CR specifications listed below, a corresponding web-platform-tests PR
must be provided, except if testing is not practical; for other
specifications it is usually appreciated. Typically, both PRs will be
merged at the same time. Note that a test change that contradicts the spec
should not be merged before the corresponding spec change.
[...]
]]
https://github.com/w3c/csswg-drafts/blob/master/CONTRIBUTING.md

Good idea? Objections? Respond on list and we can talk about it on our next
call (Sept 20).

-Dan Veditz