#TIL, (and I cannot find documented), but can easily replicate is that when a #WebComponent#CustomElement's `connectedCallback()` is called, `innerHTML` is undefined, UNLESS, you wait to define your custom element after DOMContentLoaded.
I wish there was a callback for a custom element that amounted to “the element has been loaded", but I can see that this doesn't necessarily make sense.
[Can't recreate in codepen since it runs all stuff after DOM loaded.]
https://gist.github.com/davetron5000/255ff81851c18a220fa73b329cd8ed4d