Let's say we have an html5 custom element that extends the HTMLAnchorElement and goes by the name 'custom-a', then we insert it directly in the html like this

<a href="" is="custom-a">Hello World</a>

as expected this will trigger the connectedCallback(), now how do I accomplish the same thing by creating and inserting the custom element via javascript, I tried:

<script>
        let elem = document.createElement('a');

        elem.setAttribute('is', 'custom-a');

        document.body.appendChild(elem);
</script>

But connectedCallback() fails to run