Who says you're doing JSON->HTML ? Both HTML and your data format need to get deserialized to DOM - depending on use case it can be faster/smaller to do the des

Who says you're doing JSON->HTML ? Both HTML and your data format need to get deserialized to DOM - depending on use case it can be faster/smaller to do the deserialization in JS from some format rather than using browser HTML parser. HTML is actually non-trivial to parse and isn't really optimized for anything.