HTML (Hyper-Text Markup Language) is a specific kind programming language, a markup language. It is nowhere close to a general purpose (Turing complete) programming language. Maybe you meant the web stack (HTML, JavaScript, CSS)?
HTML5 doesn't refer specifically to HTML, rather it's "HTML + JavaScript + CSS". There's hardly a website nowadays that can be used without JS.
By using the name HTML5 as an umbrella name for a programming language API plus some declarative bits, the W3C tacitly admitted that the declarative push has been a mistake - I explained "why" in my previous comment.
The fact is, JavaScript is ubiquitous in web programing and the web is unthinkable without it, it's so pervasive on the front end that it's been repurposed for backend use precisely because it makes a lot of sense to use one programming language for one project. Pure HTML is unsuitable for backend use and it sticks like a sore thumb in the front end.
Younger developers are likely missing on the history of the entire web endeavor but in order to get a feel for it, I'd recommend to focus on practical meaning rather than on trivial semantics.
The big point I was driving at is this: The damage has been done, instead of a clean rendering API we've got this hybrid monstrosity which locked us to a couple / triple browser rendering engines which, due to cutesy BS, are complex beyond comprehension, beyond security analysis or normal competition. This is what an obsession with fancy names leads to.