My thanks to @TylerH for providing a link to a similar question here on Stack Overflow where I found my answer:

...modern browser seem to not complain if I use custom tag names, and then style those tags as if it where normal HTML tags, they behave like span elements, and like div elements, if I set display: block;...

Also, as @Mr Lister commented in said question:

So if you must use custom elements, use names starting with x-.

So (for the example I provided in my question) the following HTML and CSS will be valid (in modern browsers at least)!

<!-- HTML -->
<x-something>
    <!-- Content -->  
</x-something>

And:

/* CSS */

x-something {
    /* Styling */  
}