title: Is there a way to specify an HTML5 custom element should be used only once per document?
body: <p>I searched around on Google, here on StackOverflow, probably-now-outdated HTML5 spec's, and have not found an answer.  I feel as though I'm missing something obvious.</p>
  
  <p>I'm wondering if there is a way to specify when creating an HTML5 custom element, that users of that new element <em>should</em> (or must, to be 'valid' to the element's spec) only use it once per document?</p>
  
  <p>For example with HTML's elements, 'head', 'body', 'main', etc., should only be used once within a document.  I have not been able to find a way to do this with custom elements.  Is this possible, either with vanilla HTML5, Polymer, or some other means?</p>
  
  <p>Thanks to any who can help.</p>