ha-frontend/src/html/_js_base.html.template

    <script>
      function _ls(src) {
        var doc = document.documentElement;
        var script = doc.insertBefore(
          document.createElement("script"),
          doc.lastChild
        );
        script.defer = true;
        script.src = src;
        return script;
      }
      window.Polymer = {
        lazyRegister: true,
        useNativeCSSProperties: true,
        dom: "shadow",
        suppressTemplateNotifications: true,
        suppressBindingNotifications: true,
      };
      if (!("customElements" in window &&
            "content" in document.createElement("template"))) {
        document.write("<script src='/static/polyfills/webcomponents-bundle.js'><"+"/script>");
      }
      var isS101 = /\s+Version\/10\.1(?:\.\d+)?\s+Safari\//.test(navigator.userAgent);
    </script>