[ayushnix]

Commonmark with a few essential pandoc extensions like attributes and pipe tables.

Unfortunately, Commonmark and Markdown have a widespread ecosystem and are recognized everywhere. There are commonmark preview tools available even for the terminal (glow, mdcat). Its ecosystem makes Commonmark a necessary evil, at least for me. You can also find literate programming tools for Commonmark. You can edit Commonmark in lots of different editors and many static site generators work primarily on Commonmark (Mkdocs Material, Hugo, Zola).

The only other viable alternative I know of is Emacs Org Mode. However, OrgMode is tied to Emacs and you'll have to be comfortable with Emacs to use OrgMode. There are some addons for editors like Neovim but they're not at feature parity and probably never will be because of the inherent limitations of a terminal (showing multiple different monospace fonts, for example). Emacs can generate HTML using Org documents. There's ox-hugo for the Hugo static site generator.