Edit in JSFiddle

define('river.grammer.markdown',function(exports,require,module){
  function markdown(str,scope,element){
    scope.onchange('input',function(value){
      element.innerHTML = marked(value);
    });
  }
  exports = module.exports = markdown;
})

<div id="editor" scope>
   <textarea jbind="input" placeholder="input anything with markdown grammar,such as #hello">{{input}}</textarea>
   <div markdown>{{input}}</div>
</div>

html, body, #editor {
  margin: 0;
  height: 100%;
  font-family: 'Helvetica Neue', Arial, sans-serif;
  color: #333;
}

textarea, #editor div {
  display: inline-block;
  width: 49%;
  height: 100%;
  vertical-align: top;
  -webkit-box-sizing: border-box;
  -moz-box-sizing: border-box;
  box-sizing: border-box;
  padding: 0 20px;
}

textarea {
  border: none;
  border-right: 1px solid #ccc;
  resize: none;
  outline: none;
  background-color: #f6f6f6;
  font-size: 14px;
  font-family: 'Monaco', courier, monospace;
  padding: 20px;
}

code {
  color: #f66;
}