Markdown

m.js

h = document.getElementsByTagName('head')[0] 
m = document.createElement('script'); m.src="https://pagedown.googlecode.com/hg/Markdown.Converter.js";h.appendChild(m)
document.body.innerHTML = '<table><tr><td contenteditable></td><td></td>'
a = document.createElement('style'); a.appendChild(document.createTextNode('table {width:100%;height:100%}td{width:50%;overflow:auto}td+td{background:#eeeeec}'));h.appendChild(a)
tds=document.getElementsByTagName('td')
s = tds[0]
p = tds[1]
;(_ = function() {
  if (!("Markdown" in window)) {
    window.setTimeout(_, 100)
    return
  }
  c = new Markdown.Converter()
  s.addEventListener('keyup', function() { p.innerHTML = c.makeHtml(s.innerText || s.innerHTML.replace(/<br>/g, '\n'))})
})()