A more common alternative to counting divs would be CSS classnames or (for unique elements on the page) IDs. You'd do `document.querySelector('.my-class')` to locate `<div class="my-class">` or similar, rather than using the fact that e.g. something is nested 3 divs inside <body>.

Even if this custom element trick didn't work, I don't see why one would need to count divs (at least if you control the markup, but if not then made-up tags aren't an option anyway). The article even mentions using class names as an option.