code: function isCustomElement(element) {
    assertType(element, Node, false, 'Invalid element specified');
  
    let is = getAttribute(element, 'is');
    let tag = element.tagName.toLowerCase();
  
    if (is && (getElementRegistry(is) !== undefined)) return true;
    if (tag && (getElementRegistry(tag) !== undefined)) return true;
    return false;
  }
docs: Verifies that the specified element is a custom Requiem element.
  
  @param {Node} element - Target element.
  
  @return {boolean} True if recognized as custom Requiem element, false
  otherwise.
  
  @alias module:requiem~helpers.isCustomElement