Using CSS attribute selectors, you'd need to escape the colon :
by a leading backslash\
, as follows:
description[xlink\:href="http://book.com/images/HPotter.gif"] {
background-color: gold;
}
<?xml version="1.0" encoding="UTF-8"?>
<bookstore xmlns:xlink="http://www.w3.org/1999/xlink">
<book title="Harry Potter">
<description
xlink:type="simple"
xlink:href="http://book.com/images/HPotter.gif"
xlink:show="new"> ... </description>
</book>
<book title="XQuery Kick Start">
<description
xlink:type="simple"
xlink:href="http://book.com/images/XQuery.gif"
xlink:show="new"> ... </description>
</book>
</bookstore>