In Lesson: HTML Basics: Fundamentals of HTML: Adding Images, in UNIT 1, it says: "This tag is a bit different from the others. Instead of putting the content between the tags, you tell the tag where to get the picture using src. It's also different because there is no ending tag. It has / in the tag to close it."
I am aware that you don't need the slash in HTML and that you use it in XHTML and XML. I haven't really used XML much, so I'll take your word for it. But all through the exercises, after the page Adding Images, the slash has been used. Maybe for backward compatibility for XHTML? Anyway, my point being, when the examples have used it, and in the mentioned example do not use it, please give some kind of explanation as to why in the text. Or, if it is optional in HTML5, say so, in the text.