You'll need to include two pollyfils before you include a code with your custom elements:
• webcomponents/webcomponentsjs/custom-elements-es5-adapter.js - this is for new browsers in order to understand ES5 syntax
• webcomponents/custom-elements/custom-elements.min.js - this is for old browsers without customElements
support
You can add them to your index.html
file in the following way:
<div id="custom-elements-adapter">