Earlier quoted context omitted.
Name another way of invisibly embedding a JSON object in HTML in every browser without requiring CSS.
It wouldn't be JSON, but semantic markup would be another option. (Not that I agree with the GP here, just saying.)
That allows markup for the regular html part of the mail to double as structured data for automation. I can't really decide what I prefer – this allows deduplication but gets complicated when the presentation doesn't follow the exact structure of the object you want to describe.