I also saw this at https://lyra.horse/blog/2025/08/you-dont-need-js/#fn:3 :

> You are allowed to just make up elements as long as their names contain a hyphen. Apart from the 8 existing tags listed at the link, no HTML tags contain a hyphen and none ever will. The spec even has <math-α> and <emotion-> as examples of allowed names. You are allowed to make up attributes on an autonomous custom element, but for other elements (built-in or extended) you should only make up data-* attributes. I make heavy use of this on my blog to make writing HTML and CSS nicer and avoid meaningless div-soup. ↩

(HN filtered out a "face with heart eyes" emoji from the second example.)