#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

Using #DeclarativeShadowDOM to render placeholder content that gets replaced later as the #HTML #streams in with no #JavaScript!

https://lamplightdev.com/blog/2024/01/10/streaming-html-out-of-order-without-javascript/

This is an interesting technique, but it has limitations in that you can't close the #customElement until you've loaded the content.