JSFiddle - React, Tailwind, and code Playground
HTML
<div id="showwrap">
<div id="leftbox">
<ul>
<li id="a"> <a href="">One</a></li>
</ul>
</div>
<div id="rightbox">
<ul>
<li id="b"> <a href="">Firstname Lastname</a></li>
</ul>
</div>
</div>
CSS
.hoverClass {
background-color: pink;
}
JavaScript
document.getElementById('a').onmouseover=function(){
document.getElementById('b').className += " hoverClass";
};