Web Components are generally considered to be a set of standards, and Custom Elements is one of them. The others are Shadow DOM and HTML Templates (the <template> element).

Was wondering as well. You can just go ahead and use your own element straight away. You just won't get tag omission and other short forms, but you aren't getting those by using the questionable JS API for declaring elements (or proposed new APIs for HTML fragment parsing in browsers) either.

The use case mentioned - custom list bullets - can be implemented using CSS btw [1]. Not that I'd advocate for it; like all things CSS, it involves lookup of obscure and rare custom syntax, questionable layering a la "content:", suggestion of orthogonality that isn't really there, and browser support troubles.

[1]: https://web.dev/css-marker-pseudo-element/