Cells in THEAD > TR are now TH.

This change allows Twitter Bootstrap, typeset.css and other CSS frameworks to correctly style the rendered HTML.

Header cells should always be TH. The only exception are advisory cells within headers in which case TD is acceptable (but being Markdown a user with such needs could just enter HTML for this). An example of that latter use-case is in the HTML spec itself: http://www.w3.org/html/wg/drafts/html/master/tabular-data.html#the-thead-element . Otherwise, always the TH.

Many thanks russ, and keep up the good work.