Extending native element

mm-button-styles.html

<link rel="import" href="../polymer/polymer.html">
<dom-module id="mm-button-styles">
  <template>
    <style>
      :host {
        display: block;
        border: 2px solid red;
      }
    </style>
  </template>
</dom-module>

mm-button.html

<link rel="import" href="../polymer/polymer.html">
<link rel="import" href="mm-button-styles.html">
<dom-module id="mm-button">
  <template>
  　<style include="mm-button-styles"></style>
    <content></content>
  </template>
  <script>
    Polymer({
      is: 'mm-button',
      extends: 'input'
    });
  </script>
</dom-module>

page.html

...

<input type="submit" id="edit-submit" value="Search" class="button js-form-submit form-submit" is="mm-button" />

...