title: The extension 'amp-bind' was found on this page, but is unused. Please remove this extension."
body: <p>I have a very simple AMP page that uses <code>amp-bind</code>. However, the validator always shows this message:</p>
  
  <blockquote>
    <p>The extension 'amp-bind' was found on this page, but is unused. Please
    remove this extension."</p>
  </blockquote>
  
  <p>My page <em>does</em> use <code>amp-bind</code>. The code for the page is here:</p>
  
  <pre><code>&lt;!doctype html&gt;
  &lt;html ⚡&gt;
    &lt;head&gt;
      &lt;meta charset="utf-8"&gt;
      &lt;script async src="https://cdn.ampproject.org/v0.js"&gt;&lt;/script&gt;
      &lt;link rel="canonical" href="http://localhost/amp-bind.html" /&gt;
      &lt;title&gt;AMP bind test&lt;/title&gt;
      &lt;meta name="viewport" content="width=device-width,minimum-scale=1,initial-scale=1"&gt;
      &lt;style amp-boilerplate&gt;body{-webkit-animation:-amp-start 8s steps(1,end) 0s 1 normal both;-moz-animation:-amp-start 8s steps(1,end) 0s 1 normal both;-ms-animation:-amp-start 8s steps(1,end) 0s 1 normal both;animation:-amp-start 8s steps(1,end) 0s 1 normal both}@-webkit-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-moz-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-ms-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@-o-keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}@keyframes -amp-start{from{visibility:hidden}to{visibility:visible}}&lt;/style&gt;&lt;noscript&gt;&lt;style amp-boilerplate&gt;body{-webkit-animation:none;-moz-animation:none;-ms-animation:none;animation:none}&lt;/style&gt;&lt;/noscript&gt;
      &lt;script async custom-element="amp-bind" src="https://cdn.ampproject.org/v0/amp-bind-0.1.js"&gt;&lt;/script&gt;
    &lt;/head&gt;
    &lt;body class="main"&gt;
      &lt;h3&gt;amp-bind example&lt;/h3&gt;
      &lt;p [text]="'Hello '+username"&gt;Hello World&lt;/p&gt;
      &lt;button on="tap:AMP.setState({username: 'rodders'})"&gt;Say Hello&lt;/button&gt;
    &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>
  
  <p>Can anyone explain why this message shows, and what can I do to remove it?</p>