JSFiddle - React, Tailwind, and code Playground
HTML
<span class="comma"></span>
<span class="comma">B</span>
<span class="comma"></span>
<span class="comma">D</span>
<span class="comma"></span>
<span class="comma">B</span>
<span class="comma">C</span>
<span class="comma"></span>CSS
.comma:not(:empty) ~ .comma:not(:empty):before {
content: ", ";
}