Vuejs render delayed

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/vue/1.0.16/vue.min.js"></script>
<div id="app">
  <textarea name="test" id="textarea" cols="30" rows="10" v-model="content"></textarea>
  <hr>
  <button @click="insertTag">insert strong tag</button>
  {{{ content }}}
</div>

JavaScript

new Vue({
	el: '#app',
  data: {
  	content: 'this is the inital content data'
  },
  methods: {
  	insertTag: function(){
      this.content = this.content + '<em>this is em</em>';
    }
  }
})