@namespace a url(http://www.example.org/a);
@namespace b url(http://www.example.org/b);
@namespace html url(http://www.w3.org/1999/xhtml);
*|p, *|q, *|r, *|s, *|t{ display : block ; margin-bottom : 1em }
*|p.red, *|q, *|t { background-color : red }
div.stub *|*:not([*|title*="on ch"]) { background-color : lime }

<div class="stub">
<p title="si on chantait">This paragraph should be unstyled.</p>
<p title="si il chantait" class="red">This paragraph should have a green background.</p>
<q xmlns="http://www.example.org/a" a:title="si nous chantions">This paragraph should have a green background.</q>
<r xmlns="http://www.example.org/a" a:title="si on chantait">This paragraph should be unstyled.</r>
<s xmlns="http://www.example.org/b" b:title="si on chantait">This paragraph should be unstyled.</s>
<t xmlns="http://www.example.org/b" b:ti="si on chantait">This paragraph should have a green background.</t>
</div>