It is, definitions of HTML are widely clear: "Hypertext Markup Language (HTML) is the standard markup language for documents designed to be displayed in a web browser." If a document has interactivity, that doesn't mean it will lose its definition.
That's why I suggested maybe HTML isn't the right language for defining the initial tree state of a web app these days, but it's what we've got so it's what we use. I think web apps, and web pages too to some extent, have outgrown HTML.
