JSFiddle - React, Tailwind, and code Playground

HTML

<div><span class='span1'>content1</span></div>

<div><span class='span2'>content2<em>newcontent2</em></span></div>

CSS

.span1:after {
    content:"newcontent1";
    color:red;
    display:block;
}

.span2 em {
    color:red;
    display:block;
}