I'd like to get your position on assigning documents to origin-keyed agent clusters by default. This would effectively deprecate document.domain setting, and make it available only as an opt-in feature.
Details: We'd like to change the treatment of Origin-Agent-Cluster so that the default - an absent or malformed header - would be treated as enabling origin-keyed agent clusters. This would turn the Origin-Agent-Cluster:-header from an opt-in into an opt-out feature. As a consequence, browsers could origin-isolate more pages.
The developer-visible consequence is that modifying document.domain in order to relax same-origin restrictions, which is already deprecated, will turn into an opt-in feature that must be explicitly requested.