Not to mention, what are they expecting the client to do with the header? Block every request that exceeds some particular CO2 budget? (after it already happene

Not to mention, what are they expecting the client to do with the header? Block every request that exceeds some particular CO2 budget? (after it already happened of course). Have fun with a broken, partially loaded site because a random fraction of the requests don't come through. And that's assuming that no user disables the filter because they really really need to see those cat pictures (like they do with MITM'ed TLS connections) and that servers are always truthful and don't just put a zero everywhere (which would save the CO2 otherwise spent on calculating the estimated CO2 emissions though).