<style>
  span:not(:empty) + span:not(:empty):before {
    content: ', ';
  }
</style>
<span>+7 (985) 171-71-71</span><span>test@mail.mail</span>

<!--
  Outputs to ===> +7 (985) 171-71-71, test@mail.mail

  It works even for more than 2 elements.
-->