Markdown is a superset of HTML, inheriting its multiline comment syntax. HTML doesn't have a single-line comment, and nether does any variant of Markdown (that I know of), but JavaScript -- which also uses HTML's multiline comment syntax -- does.
I propose that it would fit both the spirit of the ecosystem and the function of the language to add JavaScript's // as a single-line comment syntax, and would be a meaningful quality of life addition to users.
Additionally, the other popular choice for this across other languages, #, isn't an option due to its existing use for headers.
(Up front, I do realize that it would require parsers to look first for whitespace, or really anything other than URL schemes, but while I recognize the tradeoff I at least believe that it's worth consideration.)
Side question, while I'm here -- is anyone at Tangled interacting with existing standards, like the CommonMark team (or even potentially contributing any of its enhancements back to downstream flavors like GFM)? Since this is both kind of a blank slate to try new things and a fragmentation risk anytime we talk about doing so.