element-hidden-by-z-index.html
<!DOCTYPE html>
<meta charset="utf-8" />
<title>Element hidden by z-index</title>
<style>
* { position: relative }
#overlay,
#hidden {
height: 50px;
width: 50px;
}
#overlay {
background: blue;
z-index: 1;
}
#hidden {
background: red;
top: -50px;
z-index: -1;
}
</style>
<div id="overlay"></div>
<div id="hidden"></div>