There is at least one way, but it's not a very good one. You could just poll the element for changes like this:
var previous_style,
poll = window.setInterval(function()
{
var current_style = document.getElementById('target').style.display;
if (previous_style != current_style) {
alert('style changed');
window.clearInterval(poll);
} else {
previous_style = current_style;
}
}, 100);
The DOM standard also specifies mutation events, but I've never had the chance to use them, and I'm not sure how well they're supported. You'd use them like this:
target.addEventListener('DOMAttrModified', function()
{
if (e.attrName == 'style') {
alert('style changed');
}
}, false);
This code is off the top of my head, so I'm not sure if it'd work.
The best and easiest solution would be to have a callback in the function displaying your target.