Sure, the markdown AST is its own thing, but no doubt its a isomorphic to an obvious subset of HTML and that's not a coincidence. Moreover, you are missing Hil

Sure, the markdown AST is its own thing, but no doubt its a isomorphic to an obvious subset of HTML and that's not a coincidence. Moreover, you are missing Hillel's second point, which is that there is nothing like an "abstract node" syntax which can be used for custom extensions. Lisp has (NAME ...) and XML has <NAME>...</NAME>, and these are separate from the choice of NAME. (GitHub-flavored) Markdown's one extensible thing is ```NAME ``` but this isn't so useful because the inside is unstructured text (on purpose). You want a custom node which doesn't escape everything within, so you can have more regular or custom nodes on the inside.