>Optionally, a "/" character, which may be present only if the element is a void element.

>There is absolutely no difference between <br> and <br />.

>Actually, one might argue that adding / to a void tag is an ignored syntax error.

>every browser and parser should not handle <br> and <br /> any differently

If it's optional and has absolutely no effect and makes no difference, how exactly would one argue that it's an error?

To me, this is like saying `print ${SHELL}` is erroneous because the braces don't do anything and `print $SHELL` does exactly the same thing. It may be superfluous, but it's not erroneous.