title: Custom Polymer element with table as a base element
body: <p>I am trying to create two custom Polymer elements: tree-grid and tree-grid-item. The tree-grid would be an empty table that would accept tree-grid-item elements. Something like: </p>
  
  <pre><code>  &lt;dom-module id="tree-grid"&gt;
       &lt;template&gt;
          &lt;style&gt;
            :host {
              display: block;
            }
          &lt;/style&gt;
          &lt;table&gt;
            &lt;tbody&gt;
            &lt;content&gt;&lt;/content&gt;
            &lt;/tbody&gt;
          &lt;/table&gt;
        &lt;/template&gt;
        &lt;!-- javascript --&gt;
        &lt;/dom-module&gt;
  </code></pre>
  
  <p>and the tree-grid-item is a table row with an icon: </p>
  
  <pre><code>   &lt;dom-module id="tree-grid-item"&gt;
        &lt;template&gt;
          &lt;style&gt;
            :host {
              display: block;
            }
            .grid-item {
              border-bottom: 1px solid black;
              border-top: 1px solid black;
              border-collapse: collapse;
          }​
              &lt;/style&gt;
  
        &lt;tr class="grid-item"&gt;
          &lt;td&gt;&lt;iron-icon icon="icons:chevron-right"&gt;&lt;/iron-icon&gt;&lt;/td&gt;
            &lt;td&gt;
              &lt;content&gt;&lt;/content&gt;
            &lt;/td&gt;
           &lt;/tr&gt;
         &lt;/template&gt;
        &lt;!-- javascript --&gt;
      &lt;/dom-module&gt;
  </code></pre>
  
  <p>The issue that I am facing is that the items are not added inside the table when I test the elements: </p>
  
  <pre><code>&lt;tree-grid&gt;
       &lt;tree-grid-item&gt;element 1&lt;/tree-grid-item&gt;
       &lt;tree-grid-item&gt;element 2&lt;/tree-grid-item&gt;
   &lt;/tree-grid&gt;
  </code></pre>
  
  <p>gives a Dom structure like: </p>
  
  <p><a href="https://i.stack.imgur.com/wiSlo.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/wiSlo.png" alt="enter image description here"></a></p>
  
  <p>Obviously, the tree-grid-items should be inside the table, but Polymer just refuses to put them there. is this a common Polymer issue? is there a way around it ? Thanks</p>