title: HTML Custom Elements in firefox/edge
body: <p>I'm well aware that the <b>custom elements</b> feature can be enabled manually by setting <code>dom.webcomponents.enabled</code> and <code>dom.webcomponents.customelements.enabled</code> to true in the configuration (in Firefox at least), but can anyone help me out as to explain why the use of custom elements is encouraged in almost all documentation (w3, mozilla, etc), but only <b>Chrome</b> seems to be supported this feature properly.</p>
  
  <p>Lately I've been using this polyfill to get customElements to work on other browser:</p>
  
  <pre><code>defineElement = function( tag, opt ){
      if( typeof customElements !== 'undefined' ){
          customElements.define( tag, opt );
      } else {
          document.registerElement( tag, opt );
      }
  }
  class HTMLxSearchElement extends HTMLElement{
      constructor(){
          super();
          console.log('super');
      }
  }
  defineElement( 'x-search', HTMLxSearchElement )
  </code></pre>
  
  <p>But now the <code>document.registerElement</code> function has also been removed! Does anyone have any alternatives to get this functionality back up and running?</p>