That's only a hard requirement if you're creating a web component. Unknown elements are treated as inline elements by default, so if you're just using it for styling and using global attributes (even JS ones like onclick), you can use whatever tags you want. You only run into trouble when you start doing more advanced stuff with your non-standard elements.

https://html.spec.whatwg.org/multipage/custom-elements.html#...

> This is used for namespacing and to ensure forward compatibility (since no elements will be added to HTML, SVG, or MathML with hyphen-containing local names going forward).

So things that work today without a dash might break in the future if <badge>, for example, becomes a standard HTML element.