Add a syntax to allow comments that can be stripped before passing the output to the parser

write a routine (that would be separate from MultiMarkdown) to download linked images, save them to a tmp directory, and then convert them for use within a pdf.

Decide on appropriate management of alignment when a cell spans multiple columns. Currently, the alignment of the first cell is used. (If Markdown goes to a whitespace-based alignment option, that could be used in this instance.)

Consider whether there is a reasonable syntax for table cells that span multiple rows.

Consider a syntax for superscripts (this has been discussed before) - could convert it to MathML syntax? Or just use math markup instead as described in [Superscripts].

Certain markup gets processed within headers and shouldn't, e.g. <img>

Consider whether to incorporate the definition list syntax into a footnote to specify a glossary entry (or perhaps even without the footnote), or whether to leave well enough alone.