HTML has evolved over the years from being quite simple to being more complicated. HTML5 is just the latest name given to that long evolution. In todays jargon, HTML is largely synonymous with saying HTML5. There really is no such thing as learning generic HTML, and there’s no point in focusing on previous versions of HTML since some of the code has been deprecated, as in no longer part of the official W3C standards. That said, there are certain capabilities built into HTML5 that are arguably more immediately important and useful than others, so start with the basics and gradually learn more.
And for what it’s worth, you can’t really learn HTML5 without also learning CSS3 — they’re different things, but they go hand-in-hand, and you can’t really use one without the other.