我正在创建一个聚合物元素。我已经制作了模板，现在正在编写脚本。出于某种原因，document.querySelector 对类和 id 选择器都返回 null。不确定这是否不适用于聚合物（没有理由它不应该）或者我没有导入某些东西或者其他什么问题。

event-card.html

<link rel="import" href="../components/polymer/polymer.html">
<link rel="import" href="event-card-description.html">

<polymer-element name="event-card" attributes="header image description">
  <template>
    <style>
      .card-header {
        display: block;
        position: static;
        font-size: 1.2rem;
        font-weight: 300;
        width: 300px;
        height: 300px;
        border-radius: 50%;
        -webkit-border-radius: 50%;
        -moz-border-radius: 50%;
        overflow: hidden;
      }
      event-card-description {
        margin: 0;
        position: relative;
        top: 225px;
      }
    </style>

    <div 
      style="background: url(../images/{{image}}.jpg) no-repeat; background-size: cover" 
      class="card-header" layout horizontal center
      on-mouseover='{{onHovered}}' 
      on-mouseout='{{onUnhovered}}'>
      <event-card-description
        id="description"
        header={{header}} 
        description={{description}}>
      </event-card-description>
    </div>
  </template>

  <script>
    Polymer('event-card', {
      onHovered: function() {
        var elem = document.querySelector("#description");
        console.log(elem);
      }
    });
  </script>
</polymer-element>