JSFiddle - React, Tailwind, and code Playground

HTML

<div id="box">
</div>

<form id="test_form" action="">
    <label>Name: </label><input type="text" name="name" /><br/>
    <input type="submit" value="Submit" />
    </form>

CSS

#box {
    width: 50px;
    height: 50px;
    background-color: blue;
    position: absolute;
    display: none;
    color: white;
    text-align: center;
    font-size: 11px;
}

#test_form {
    margin-left: 150px;
}

JavaScript

(function() {
    var box = $('box');
    
    document.addEvent('mousemove', function(e) {
        box.setStyles({
            top: e.page.y,
            left: e.page.x
        });
    });
    
    $('test_form').addEvent('submit', function(ev) {
        ev.stop();
        box.setStyle('display', 'block');
        var sizes = box.getPosition();
        box.set("html", [sizes.x, ' x ', sizes.y].join("<br/>"));
    });
})();