How to create moving tooltip near cursor on a certain DIV without jquery

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Javascript

HTML

As title. I am maintaining a web page system. I got a request that some tooltips are needed when cursor moves onto some element(dynamically created using JavaScript). I've
tried to use some ways to solve it, and it works well on very simple pages but no effect in my system. Could someone tells me what I am missing and how to solve it?
(Sorry for no jQuery because my system doesn't support this now.)

What I have tried:

AppendedElement = (AppendedElement != undefined) ? AppendedElement : document.getElementById(AppendedElementId);
        var Span = document.createElement("span");
        Span.style.display = "none";
        Span.style.position = "relative";
        Span.innerHTML = ToolTipText;
        AppendedElement.appendChild(Span);
        AppendedElement.addEventListener("mouseover", function () {
            Span.removeAttribute("style");
        });
        AppendedElement.addEventListener("mouseout", function () {
            Span.style.display = "none";
        });