JSFiddle - React, Tailwind, and code Playground

HTML

<p class="para">This is a paragraph, yo.</p>
<p>And this here <em>is inside a paragraph</em>, about paragliders.</p>

CSS

.highlighted { background:yellow }

JavaScript

highlightWord(document.body,'para');

function highlightWord(root,word){
  textNodesUnder(root).forEach(highlightWords);

  function textNodesUnder(root){
    var walk=document.createTreeWalker(root,NodeFilter.SHOW_TEXT,null,false),
        text=[], node;
    while(node=walk.nextNode()) text.push(node);
    return text;
  }
    
  function highlightWords(n){
    for (var i; (i=n.nodeValue.indexOf(word,i)) > -1; n=after){
      var after = n.splitText(i+word.length);
      var highlighted = n.splitText(i);
      var span = document.createElement('span');
      span.className = 'highlighted';
      span.appendChild(highlighted);
      after.parentNode.insertBefore(span,after);
    }
  }
}