The trailing slash itself is not invalid, per se, but it'a very hard for any following markup to be valid unless the element is void. A particularly nasty one

The trailing slash itself is not invalid, per se, but it'a very hard for any following markup to be valid unless the element is void. A particularly nasty one is any attempt to use a supposedly self-closing script tag: <script src="..." /> This results in the rest of your page being interpreted as code. Oops!