pkg/polymer/example/scoped_style/my_test.html

<polymer-element name="my-test">
  <template>
    <style>
      p { color: red;}
    </style>
    <p>Inside element, should be red</p>
  </template>
  <script type="application/dart;component=1">
    import 'package:polymer/polymer.dart';

    @CustomTag('my-test')
    class MyTest extends PolymerElement {}
  </script>
</polymer-element>