highlight.js

https://js-cqdfne.stackblitz.io/

highlightNode(document.body, 'angular');

function highlightNode(root, word) {
    const textNodes = textNodesUnder(root);
    console.log('Found nodes: ' + textNodes.length, textNodes);

    const matchingNodes = textNodes
            .filter((node) => new RegExp(word, 'gi').test(node.nodeValue))
            .map((node) => [node, new RegExp(word, 'gi').exec(node.nodeValue)]);
    console.log('Matching nodes: ' + matchingNodes.length, matchingNodes);
}


function replaceWord(node, word) {
    const highlightTag = 'em';
    const highlightClassName = 'highlighted';

    let match = document.createElement(highlightTag);
    match.className = highlightClassName;
    match.appendChild(document.createTextNode(word));
    match.style.backgroundColor = 'yellow';

    var after = node.splitText(regs.index);
    after.nodeValue = after.nodeValue.substring(regs[0].length);
    node.parentNode.insertBefore(match, after);
}


function textNodesUnder(el){
  var n, a=[], walk=document.createTreeWalker(el,NodeFilter.SHOW_TEXT,null,false);
  while(n=walk.nextNode()) a.push(n);
  return a;
}