I searched around on Google, here on StackOverflow, probably-now-outdated HTML5 spec's, and have not found an answer.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.
I'm wondering if there is a way to specify when creating an HTML5 custom element, that users of that new element should (or must, to be 'valid' to the element's spec) only use it once per document?
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?
Thanks to any who can help.