Pushing HTML even further, one could say it’s a declarative programming language that programs a UI in a mostly-stateless manner (inputs aren’t really stateless but you can argue the state is provided by the UI rather than managed by HTML).

I’m not sure I’d make this leap myself though, I have a hard time classifying it (or any other markup language) as a PL. As far as I am aware, you can’t really program a state machine with pure HTML, though you can accept inputs and return outputs at least.