Yes it will. You can help yourself by using the w3's validator direct input option:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<head>
<title>Title</title>
</head>
<body>
<p>
<span>Test<span>Nest span</span></span>
</p>
</body>
</html>