gistfile1.html

<template instantiate>
  <import tags="my-button" from="source"></import>
  
  <!-- the template is never actually instantiated until the imports have loaded. -->
  <my-button></my-button>
</template>

<!-- crazier, more fleshed out stuff -->
<template instantiate>
  <import tags="my-button" from="source">
    Fallback content while loading
  </import>
</template>

<template instantiate>
  <import>
    <tags named="my-button" from="source"></tags>
    <tags named="my-textarea" from="othersource"></tags>
    <loading>
      Fallback content while loading
    </loading>
    <error>
      Fallback content when error
    </error>
  </import>
</template>