What we're discussing are not custom elements. If you rename a div into <some-other-name>, it doesn't make that a custom element.
Very quite literally in the very literal sense of the word the only way to create actual custom elements is through javascript. There's no other way.
A sibling comment posted this link: https://codepen.io/dkoch-org/pen/yLwPVYG. And this is not custom elements. These are "elements unknown to the browser which will be treated in a neutral way making them indistinguishable from divs/spans". In the context of "preventing div/span soup" this does absolutely nothing. It's the same soup, just with longer names.
Defining them in js doesn't "do anything" either. I think the dispute here boils down to why you would prefer longer names over div/span everywhere. It doesn't make any difference to users, but can make it a lot easier for developers. That's the main reason to do it for me.