pecific elements can be registered via <script> tags at the
// top of individual documents.
Services.obs.addObserver(
  {
    observe(doc) {
      if (
        doc.nodePrincipal.isSystemPrincipal &&
        (doc.contentType == "application/xhtml+xml" ||
          doc.contentType == "text/html") &&
        // People shouldn't be using our built-in custom elements in
        // system-principal about:blank anyway, and trying to support that
        // causes responsiveness regressions.  So let's not support it.
        doc.URL != "about:blank"
      ) {
        Services.scriptloader.loadSubScript(
          "chrome://global/content/customElements.js",
          doc.ownerGlobal
        );
      }
    },
  },
  "document-element-inserted"
);
PK