Ah sorry, I didn't see. So the decision about DOM elem vs String is one I'm not 100% sure about, but I think the performance difference is at most like a 10-15% difference which wasn't that compelling to us. So I didn't see a strong reason to go one way or the other and the code I believe is cleaner producing nodes rather than Strings, but I easily could be wrong on both counts.
I didn't actually consider EDN. I haven't see enough of a win over JSON to consider it, but that could just be misinformed.