Apart from the validity issue, which simply depends on the doctype you are validating against, the slash really matters only if your page is served with an XML content type, such as application/xhtml+xml (which is rarely used, because old versions of IE choke on it).
If you do that, then well-formedness error handling is Draconian: any error (such as a start tag without a matching end tag, which can be the start tag itself when the ![]()
