Try to custom-elements-es5-adapter
to your imports or html (depending on your needs).
Docs for using custom-elements-es5-adapter.js
import '@webcomponents/webcomponentsjs/custom-elements-es5-adapter.js';
Stackblitz
Further examples:
using script src
in html: Stackblitz
<!DOCTYPE html>
<html lang="en">
<head>
<script src="./node_modules/@webcomponents/webcomponentsjs/custom-elements-es5-adapter.js"></script>
<script src="./node_modules/@webcomponents/webcomponentsjs/webcomponents-loader.js"></script>
</head>
using import custom-elements-es5-adapter.js
: Stackblitz
// These imports must be first.
import '@webcomponents/webcomponentsjs/webcomponents-bundle.js';
import '@webcomponents/webcomponentsjs/custom-elements-es5-adapter.js';
// All other top-level imports should be specified here so that (a) they follow
// the polyfills and adapter and (b) so that stackblitz will convert their
// module specifiers.
import '@polymer/iron-selector/iron-selector.js';
https://stackblitz.com/edit/js-woikq4?file=index.js