According to [1], Riot developers are aware of that and chose to support custom tags without a dash:
> W3C specification demands you use a dash in the tag name. Instead of <person> you must write <my-person>. Obey this rule if you care about W3C. Both work fine.
However using hyphens to be standards-compliant and future compatible would be better.
[1]: https://muut.com/riotjs/faq.html