Keeping hints about alternate syntax used in the AST

Spec
3

That’s what I assumed.
The delimiters are not directly supported in HTML, though, just with CSS. But if we take that into account, there‘s also a case for the bullets which could be correlated with presentational HTML 3/4, e.g. * → disc, + → circle, - → square.
If I remember correctly, Latex does not support item styles for enumerate environments out of the box, i.e. a package like enumitem or enumerate was needed or each item’s label would have to be generated explicitly (including the number).

If a hint about actual marker used was available in the AST, someone could write filters or extensions that did something more reasonable like the following:

  • Treat underscore emphasis as i and b in HTML5 output,
    maybe even u (or ins?) for quadruple underscores.
  • Auto-number ATX headings, don’t number Setext headings or
    don’t put Setext headings into an automatically generated TOC or
    generate HTML5 sections for ATX headings only or
    do any combination thereof.
  • Treat one or two of the “thematic break” alternatives as section boundaries in HTML5 output.
    (@Dmitry once suggested to use “boundary” instead of “horizontal rule” or “thematic break”.)
  • Render images or formatted text for certain code blocks with tilde fences, but restrict to syntax-highlighting for back-tick fences. They could also differ just in default values for the parser-dependent info-string “attributes”, e.g. preset language, line numbers, whitespace visibility.