org-mode already has this. Unfortunately, people seem to continue to use the inferior version of org known as Markdown. It is somewhat like how pdf overtook the superior format djvu.
I agree that Markdown is probably an inferior markup format compared to org but serious usage of org is tied to Emacs. Moreover, Markdown (or CommonMark) has an extremely healthy ecosystem and a lot of tools that make life easier, unlike org mode. The most popular static site generators also use Markdown and RST.
Unless using Emacs becomes a user friendly beginner's choice (which it isn't, and that's fine) or Org is able to distinguish itself in isolation from Emacs, I don't see org being widely used anytime soon. If org gains first class support in editors like neovim, vscode and tools like pandoc, mdcat etc, I could see myself using it.
Richard P. Gabriel (who every Emacs person should know) wrote "Worse is Better"[1] in 1984, which is directly applicable. To summarise, "worse" software prioritises simplicity of implementation and priortises simplicity over correctness.
yes. markdown is html as org-mode, which suitable for web content. web page normally has some add-on components, e.g. header, menu, theme, this is a try to handle them within markdown.
