JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li class="class1">A</li>
    <li class="class1 class2">B</li>
    <li class="class1">C</li>
</ul>
<ul>
    <li class="class1 class2">D</li>
    <li class="class1">E</li>
    <li class="class1">F</li>
</ul>

CSS

li.class1:first-child:not(.class2) {
    background-color: red;
}