[css-selectors] Tagname based substrings

Hi all,
In my ongoing investigation of web components, I'm finding a place where I
want to target a substring of a tagname to keep the authoring semantic.
That is, when I author my tags like this:
<x-navigation />
<x-quiz />
etc...

I want to namespace to them so they have shared attributes like fonts, etc.
I can't find a way in the documentation on attribute substrings, but might
just be missing it. Is there a way to target a tagname? I'm thinking like
this:

*[tagname^="x-"] { ... }

Thanks,
-Greg