JSFiddle - React, Tailwind, and code Playground

CSS

b {color:red}

JavaScript

var string1 = "DD,CC,FF";
var string2 = "FF,XX,DD";

var new_string = checkStrings(string1, string2);

document.body.innerHTML = new_string;


function checkStrings(str1, str2) {
    str1 = Array.isArray(str1) ? str1 : str1.split(',');
   
    for (var i=0; i<str1.length; i++) {
        if (str2.indexOf(str1[i]) == -1) {
            str1[i] = '<b>' + str1[i] + '</b>';
        }
    }
    return str1.join('');
}