What’s the right way to spell “HTML5”? The short answer is: “HTML5” (without a space).
People in the WHATWG community have commonly referred to HTML5 as “HTML5” for quite a while. However, when the W3C HTML WG voted on adopting “Web Applications 1.0” the question about the title said “HTML 5”. Thus, the W3C HTML WG voted to adopt “HTML 5” as the title, but it wasn’t a vote for or against the space but about “HTML” and “5” in contrast to e.g. “Web Applications 1.0”. Anyway, as a result, the spec was retitled literally “HTML 5”.
This lead to inconsistency. Sometimes people kept writing “HTML5” and sometimes “HTML 5” (even on whatwg.org). This kind of inconsistency is bad for branding. The Super Friends pointed this issue out as the first thing they pointed out.