What version of HTML is used in the lessons?
HTML 4 and HTML5. Valid HTML 4 is (with some exceptions) valid HTML5. It's still the same language, only with some new elements added in HTML5. The HTML language is a 'living language' subject to new recommendations going forward as technology advances.
If you see something like this,
<img />, that is a carry over from XHTML, which is also valid HTML5, only XML compliant. HTML5 needs XHTML standard markup to be XML compliant. This only applies if the pages are being served with the mime type,