It's not possible to do with CSS. You can either use a workaround in CSS or JS to do it.
I think of all the available options these are the most common: data attributes (especially for utility/component styling), IDs if unique, or just adding an additional class in this case.
There are lots of options. Here are a few examples:
/* green for all */
a[rel="category tag"] {
color: green;
}
/* override only for the ones with the data-attribute */
a[rel="category tag"][data-only-me] {
color: darkblue;
}
/* override only for the ones with the extra rel */
a[rel="category tag special"] {
color: red;
}
/* override only for a specific class */
a.special {
color: hotpink;
}
/* override only for the id */
#special-snowflake {
color: orange;
}
<a class="u-textlink" href=“www.link.com" data-only-me="" rel="category tag">Sustainability</a>
<a class="u-textlink" href=“www.link.com" rel="category tag">Something else</a>
<a class="u-textlink" href=“www.link.com" rel="category tag special">Something else</a>
<a class="u-textlink special" href=“www.link.com" rel="category tag special">Something else</a>
<a id="special-snowflake" class="u-textlink" href=“www.link.com " rel="category tag ">Something else</a>