That said, I'm not sure how you'd go about server-side rendering React from Hacklang, as it seems that the open-source solution to do so is no longer supported (https://github.com/hhvm/xhp-js).
To clarify, xhp-js was not server-side rendering - it's a framework for generating javascript code, and getting an DOM element ID for a react element that the hack code can refer to and use elsewhere, e.g. by passing to some other generated JS.
That said, I'm not sure how you'd go about server-side rendering React from Hacklang, as it seems that the open-source solution to do so is no longer supported (https://github.com/hhvm/xhp-js).