To fully answer this question. Yes, self closing tags (a and div elements in this example) is perfectly valid according to the EPUB spec. The following example conforms to the EPUB 2 spec.
Code:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title/>
</head>
<body>
<p>
<a id="blah" />
</p>
<div id="blah1" />
<div id="blah2" class="clearfix" />
</body>
</html>
The relevant sections of the EPUB 2 Spec are
1.4.1.2 and
Appendix A. XHTML 1.1 does allow self closing tags as used above. Specifically Appendix a requires that the document validate against the XHTML 1.1
DTD.