text/enriched has been around for decades, and supports basic font styling (bold/italic/underline, color, font face, font size) and that's basically it.

Actually, text/markdown exists as well. However, the definition of markdown syntax is, um, less than precise: https://daringfireball.net/projects/markdown/syntax (the text/markdown RFC literally has a parameter to indicate which flavor you meant by markdown!). And it incorporates HTML too, FWIW--legal HTML fragments are legal markdown as well. Honestly, markdown's million variants makes the HTML support landscape look uniform.