After spending many hours trying to understand why my attempts to get a RESTful sensor to collect data from my emonitor was failing I think I’ve figure it out; it is sending a header " Content-Type: text/html". The payload appears to be perfectly fine XML and which Home Assistant could convert into JSON. Of course when it came time to try on the real thing it would fail which lead me to looking at headers.
I’ve looked at the code and it looks like if doesn’t think it is XML it will try to parse it straight as JSON.