@namespace html url(http://www.w3.org/1999/xhtml);
@namespace a url(http://www.example.org/a);
@namespace b url(http://www.example.org/b);
*|p, *|l { display : block ; margin-bottom : 1em }
div.test * { background-color : red }
div.test *:not(a|p) { background-color : lime }

<div class="test">
 <p xmlns="http://www.w3.org/1999/xhtml">This paragraph should have a green background.</p>
 <p xmlns="http://www.example.org/b">This paragraph should have a green background.</p>
 <p xmlns="">This paragraph should have a green background.</p>
 <p xmlns="http://www.example.org/a">
 <l>This paragraph should have a green background.</l>
 </p>
</div>