> If you include hyphens in the name, you can guarantee that your tag won’t appear in any future versions of HTML.

Which future version? Is there something I'm missing? I'd like to have html6 that has feature parity with native, but I'm afraid we got html5 and just stopped there. And if there will be an html6 why can't we just state "use version 5 in this document" so there won't be any name clashing between supported and custom tags?

> but I'm afraid we got html5 and just stopped there.

They dropped numbered versions. Now it is just HTML which is continuously evolved in a backwards-compatible manner.

> And if there will be an html6 why can't we just state "use version 5 in this document"

Because browser vendors are opposed to explicit versioning, since it introduces multiple rendering modes.

HTML 5 still gets new features all the time: https://github.com/whatwg/html

But they'll never add new standard tags with hyphens.