code: function stripCustomNsAttrs(el) {
      DOM.attributeMap(el).forEach(function (_, attrName) {
          if (attrName === 'xmlns:ns1' || attrName.indexOf('ns1:') === 0) {
              DOM.removeAttribute(el, attrName);
          }
      });
      for (var _i = 0, _a = DOM.childNodesAsList(el); _i < _a.length; _i++) {
          var n = _a[_i];
          if (DOM.isElementNode(n))
              stripCustomNsAttrs(/** @type {?} */ (n));
      }
  }
docs: When IE9-11 comes across an unknown namespaced attribute e.g. 'xlink:foo' it adds 'xmlns:ns1'
  attribute to declare ns1 namespace and prefixes the attribute with 'ns1' (e.g. 'ns1:xlink:foo').
  
  This is undesirable since we don't want to allow any of these custom attributes. This method
  strips them all.
  @param {?} el
  @return {?}