JSFiddle - React, Tailwind, and code Playground

HTML

<html>
    <head>
    </head>
    <body onclick="alert('preexisting handler')">
        <div id="mydiv">DIV</div>
    </body>
</html>

CSS

body { background: blue }
#mydiv { width: 50px; height: 50px; margin: 50px; background: red }

JavaScript

document.observe('click', function(e) {
    var o = e.findElement('div');
    if(!o.match('div#mydiv') && !o.up('div#mydiv'))
        $('mydiv').hide();
});