Note that we are at least 3-4 weeks away before both Parsoid and RESTBase are ready with code, storage, and API changes. At that point, we will also have to evaluate which of the clients are ready to switch over, and if not, what the performance impacts are on the Parsoid cluster (to convert from the new to the old version), and whether any of the clients have any blockers on this deployment. But, I am creating new tasks for all Parsoid HTML clients to start surfacing any issues that need to resolved for T78676: Store & load data-mw separately to be enabled in production.

https://en.wikipedia.org/api/rest_v1/?doc#!/Page_content/getFormatRevision does not seem allow to try out the new Accept header version yet.

And whether to load data-mw in CX frontend or in cxserver...

1. cxserver doesn't really need it, but...
2. Special:CX is already heavy on client side, so doing the extra load and merge in there could make it even slower
3. To make sure we have the matching versions, I would do it in cxserver
4. We need to store the merged things in the corpora table anyway with current design, so we need to merge them instead of storing separately.

In T130643#2356789, @Nikerabbit wrote:

https://en.wikipedia.org/api/rest_v1/?doc#!/Page_content/getFormatRevision does not seem allow to try out the new Accept header version yet.

We haven't yet enabled this in RESTBase and production. There are still a couple things that we need to work through on the Parsoid & RB end. This will take another couple weeks at least.