We actually require an about id and a data-mw for extension content. I just noticed there is a minor gap in our spec for the references extension (that section show the data-mw for the references block in <ol>). However, all transclusions and extensions do show data-mw.

I'll fix this.

But, we would need this for it to serialize properly.

<ol class="references" typeof="mw:Extension/references" about="#mwt2" data-mw='{"name":"references","attrs":{}}'></ol>

We should perhaps not throw a 503 and instead ignore this and generate an error message perhaps. We'll discuss that internally.

We could also perhaps tweak our serializer to infer empty attributes from the typeof and generate a self-closing empty tag. To be investigated.

But, if you your content translation engine to publish the data-mw and about attributes, it should work.

Even if the issue has been solved in the Content Translation tool by keeping data-mw, I've uploaded a patch that logs an error instead of returning 503 in this case: https://gerrit.wikimedia.org/r/#/c/176950/ .