There was an attempt to standardise what looks like an HTML-subset for formatted emails: https://en.wikipedia.org/wiki/Enriched_text https://en.wikipedia.org/wi

There was an attempt to standardise what looks like an HTML-subset for formatted emails: https://en.wikipedia.org/wiki/Enriched_text https://en.wikipedia.org/wiki/Enriched_text But unfortunately it wasn't very popular.