You can split the text and convert the keyword "FREE" to a span element. So you can style the keyword "FREE". This method is safe because does not alter any non-text html element.
var keyword = "FREE"; var headline = document.getElementById("headline"); var highlight, index; headline.childNodes.forEach(child => { if (child.nodeType == Node.TEXT_NODE) { while ((index = child.textContent.indexOf(keyword)) != -1) { highlight = child.splitText(index); child = highlight.splitText(keyword.length); with(headline.insertBefore(document.createElement("span"), highlight)) { appendChild(highlight); className = 'highlight'; } } } });.highlight { /* style your keyword */ background-color: yellow; }<div id="FREE"> <h1 id="headline">"Get your FREE toothbrush! FREE floss set and dentures!"</h1> </div>