data-markdown.js

// From https://gist.github.com/1343518
// Modified by Hakim to handle markdown indented with tabs
(function(){

  [].forEach.call( document.querySelectorAll('[data-markdown]'), function  fn(elem){
    
    // strip leading whitespace so it isn't evaluated as code
    var text = elem.innerHTML;
    
    var leadingWs = text.match(/^\n?(\s*)/)[1].length,
        leadingTabs = text.match(/^\n?(\t*)/)[1].length;

    if( leadingTabs > 0 ) {
        text = text.replace( new RegExp('\\n?\\t{' + leadingTabs + '}','g'), '\n' );
    }
    else if( leadingWs > 1 ) {
        text = text.replace( new RegExp('\\n? {' + leadingWs + '}','g'), '\n' );
    }

    // here, have sum HTML
    elem.innerHTML = (new Showdown.converter()).makeHtml(text);

  });

})();