How to highlight scrolling text

Hi..
I have the following code that scrolls some text across a textbox.
How can I highlight the current word in yellow
I can't even highlight a single char with something like this

 document.getElementById('scrolling_text_box').innerHTML = text_box += "<span class='highlight'>" + x + "</span>";

Any ideas?

Here's my code

 Scroller.prototype.scroll = function () {
        var text_scroll = this.scrollText;
        var text_box = document.getElementById('scrolling_text_box').innerHTML;
        var length_scroll = text_scroll.length;
        var length_box = text_box.length;

        this.stopped = false;

        if (length_box > this.cut_text) {
            text_box = text_box.substring(1, text_box.length);
        }

        // Letter
        var x = text_scroll.substring(this.characters, this.characters + 1);

   
        text_scroll = text_box + x;
     
          document.getElementById('scrolling_text_box').innerHTML = text_scroll;

          this.characters = this.characters + 1;

        var obj = this;
        this.timer = setTimeout(function () { obj.scroll(); }, this.speed);

        if (this.characters >= length_scroll) {
            this.stopped = true;
            clearTimeout(this.timer);
        }
    }

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