KeenWrite uses the Java-based FlyingSaucer library for rendering HTML in the preview. Moreover, KeenWrite exports Markdown as XHTML, which ConTeXt imports for typesetting into PDF. While Sheet Markdown would be a fun integration, it would have to execute prior to displaying (i.e., not use JavaScript for in-browser rendering, but use Java to preprocess the Markdown tables before inserting into the XHTML).
Thanks for the explanation. I think integrating Sheet Markup with that process would be challenging, since (as I understand it) XHTML doesn't support canvas.