A recent project of mine heavily utilizes nested custom elements with shadowroots. I tried implementing rrweb but failed. Here is a running example (implementing the getting started guide; view source recording start and recording play): http://035401c.xcenic.maximilian.mairinger.com/admin/. After clicking "Stop recording" view the recording object in the console. Everything outside the first shadowroot is properly recorded, but nothing inside (very similar to a lot of other solutions out there).

I noticed however your effort to support shadow roots and tried setting a decently low checkoutEveryNms time. Which worked, although at the cost of absurdly high cpu utilization (not surprising), proving that the pure serialization process works.