You are probably using <br> instead of <br />, <hr> instead of <hr />, <img> instead of <img />, etc.
Bassically, apply /> this to any tag that doesn't have a seperate closing tag. (don't forget the *space* before "/>")
This wouldn't apply to <a> tag because you have a speparate </a> tgat that closes it. You can use this on list items <li />. There's not a ton of tags this applies to as most have their own seperate closing tags.