"User agents must treat elements and attributes that they do not understand as semantically neutral; leaving them in the DOM (for DOM processors), and styling them according to CSS (for CSS processors), but not inferring any meaning from them."
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.
"User agents must treat elements and attributes that they do not understand as semantically neutral; leaving them in the DOM (for DOM processors), and styling them according to CSS (for CSS processors), but not inferring any meaning from them."