As the specification on W3C says, it is not possible to share any kind of a DOM API from your main thread with a worker thread.
Excerpt from the spec:
The DOM APIs (Node objects, Document objects, etc) are not available to workers [...]
Since a DocumentFragment inherits at least from Node, it should be (nearly) impossible to send it to a webworker. I said nearly, because it might be possible to send it as a string (for example via innerHTML), but that means you would have to parse it in some way inside the worker thread or gather your information from that string. Personally, I don't think there is any smart way to do it.
A possible solution might be to pass the raw data as JSON to your worker thread, construct a string there an pass that string back.