data-markdown.js

// From https://gist.github.com/1343518, modified to not load showdown
(function boom(){

  [].forEach.call( document.querySelectorAll('[data-markdown]'), function  fn(elem){
      
    // strip leading whitespace so it isn't evaluated as code
    var text      = elem.innerHTML.replace(/\n\s*\n/g,'\n'),
        // set indentation level so your markdown can be indented within your HTML
        leadingws = text.match(/^\n?(\s*)/)[1].length,
        regex     = new RegExp('\\n?\\s{' + leadingws + '}','g'),
        md        = text.replace(regex,'\n'),
        html      = (new Showdown.converter()).makeHtml(md);

    // here, have sum HTML
    elem.innerHTML = html;

  });

}());