[faboo]

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.

[yoz-y]

You can still run into problems if the standard one day decides to actually give your custom tag any semantics and default styles.