MJML is opinionated and prescriptive, to the point of being a markup language in its own right. HEML is not opinionated: You write HTML and CSS. HEML just takes

MJML is opinionated and prescriptive, to the point of being a markup language in its own right. HEML is not opinionated: You write HTML and CSS. HEML just takes care of all the arcane eccentricities of various HTML mail clients.