@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);
div.stub > * { display : block ; margin-bottom : 1em ;
                            background-color : red }
div.stub > *:not(|p) { background-color : lime }
div.stub > *|l > *:not(|p) { background-color : red }

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