Semantic in machine processing is actually very simple: if a machine has an instruction to process an element and we know what it does, then the element is semantic.

So, for example, <b> and <i> have perfect semantic, while <article> not so much. What does the browser do with an <article>? Or maybe it is there for an indexing engine? I myself have no idea (nor that I investigated that, I admit).

But all that was misunderstood, very much like XML itself.