How are you going to stop people from serving arbitrary content though? What appears to one parser as "Alice sent Bob 0.00000006767 BTC" may appear to another

How are you going to stop people from serving arbitrary content though? What appears to one parser as "Alice sent Bob 0.00000006767 BTC" may appear to another parser as malicious code. So long as there is uncensorable data anywhere, this will always remain possible.