I wanted the whole of {{content}} to be under a single easily foldable H2 heading. But content which contains H2 headings threw a wrench in my works. After lots of trial and error, I solved it with replace_tags. Saving future googlers some trouble:
{{contentHtml|replace_tags:"h6":"strong"|replace_tags:"h5":"h6"|replace_tags:"h4":"h5"|replace_tags:"h3":"h4"|replace_tags:"h2":"h3"|markdown}}
This lowers all headings one level down, and makes H6 bold instead:
| Before | After |
|---|---|
| H2 | H3 |
| H3 | H4 |
| H4 | H5 |
| H6 | Bold |
Alternatively, formatting everything as a blockquote:
{{content|blockquote}}
or as callout:
{{content|callout}}
works too–but this won’t allow to link to or embed individual blocks, which is a loss in my book.