Hi all,
Well, first of all, thanks all for CommonMark and special thanks to John MacFarlane for cmark! 
I’ve just published xmark at GitHub https://github.com/vieiro/xmark
xmark is an XSLT stylesheet that generates Tufte-CSS powered HTML from CommonMark XML AST.
Generated output may contain a Table of Contents, supports syntax highlighting, HTML figures in three Tufte-CSS formats (normal, full-width, margin), side notes and margin notes.
Example output is available at github here: http://vieiro.github.io/xmark/ (instructions included).
I thought the project could be useful for those of us wanting to manipulate CommonMark’s XML AST quickly with a simple text editor and the xsltproc XSLT processor.
Cheers,
Antonio