gistfile1.html

<!-- Element declaration -->
<polymer-element name="test-element" noscript attributes="source">
  <template>
    <img src="{{ source }}" /> 
  </template>
</polymer-element>

<!-- Usage -->
<test-element source="https://www.google.ru/images/srpr/logo11w.png"></test-element>

<!-- Emcee v0.2. Generated code. -->
<polymer-element name="test-element" noscript attributes="source">
  <template>
    <img src="{{ source }}" /> 
  </template>
</polymer-element>

<!-- Emcee v0.2. Element's shadow root. -->
<img src="https://www.google.ru/images/srpr/logo11w.png">

<!-- Emcee v1.0. Generated code. -->
<polymer-element name="test-element" noscript attributes="source">
  <template>
    <img src="%7B%7B%20source%20%7D%7D"> 
  </template>
</polymer-element>

<!-- Emcee v1.0. Element's shadow root. -->
<img src="../../../assets/%7B%7B%20source%20%7D%7D">