JSFiddle - React, Tailwind, and code Playground

HTML

<textarea id='textarea'></textarea>
<div id='result'></div>

CSS

textarea {
    width: 196px;
    height: 100px;
}
#result {
    border: 1px solid gray;
    width: 200px;
    height: 200px;
    overflow: scroll;
}
.highlighted {
    background: yellow;
}

JavaScript

$('#textarea').on('input', function() {
    var text = $('#textarea').val();
    $('#result').html(text);
    if ($('#result').html().indexOf('<span>') > -1) {
        $('span:not(.highlighted)').addClass('highlighted');
    }
});