title: What's the reason behind not allowing self-closing tag in custom element in spec?
body: <p><code>&lt;self-closing /&gt;</code> tag are not allowed in HTML custom element.</p>
  
  <p><strong>Why</strong> is that?</p>
  
  <p>Many cases just need <em>void tag</em>, I guess by that, code would be more readable. 
  On the other hand, it implies semantic meaning more directly.</p>
  
  <p>Just like <code>&lt;hr /&gt;</code>, which by code  itself means there shouldn't be any children inside it, because there's no place for that.
  Plus it has nothing to do with children.</p>
  
  <blockquote>
    <p>Custom elements cannot be self-closing because HTML only allows a few
    elements to be self-closing. (<a href="https://developers.google.com/web/fundamentals/getting-started/primers/customelements" rel="noreferrer">read more</a>)</p>
  </blockquote>