XHTML 1.0: The Extensible HyperText Markup Language (Second Edition)

C.2. Empty Elements

Include a space before the trailing / and > of empty elements, e.g. <br />, <hr /> and <img src="karen.jpg" alt="Karen" />. Also, use the minimized tag syntax for empty elements, e.g. <br />, as the alternative syntax <br></br> allowed by XML gives uncertain results in many existing user agents.
Unless I'm reading that backwards, it says to include the space. It's not necessarily required, but it is recommended.

Also, what do you wish you could believe? That I have taught and done (and am still involved in at a different level) curriculum development?

I agree that you should challenge a student. However, there is a difference between giving a kindergartener a Calculus III problem and giving a college student a critical thinking problem. You can't challenge a student if they have no information yet. Let them grasp the simple basics first, then push that information and their knowledge further with challenges. Giving a student a challenge they are prepared for and trying to give them a BS in Computer Science worth of information on day 1 are two very different things.

I've never thought a student too dumb to do what is required and I would frequently challenge my students to solve problems that I never directly taught, but that they had the ground work for... requiring them to think outside of the box.