Why won't the z-index on my elements work? - html

Question

I have my code set up so I have the hero image at the bottom and the overlay on top with the text and button in overlay. I also have the navigation bar with a z-index but for some reason the button for my resume in overlay isn't working.
HTML
<div id="header">
<a href="index.html"><div id="leftHeader">
<img src="assets/logo2.jpg" alt="Logo" style="width:65px;height:65px">
<h1>Amanda Farrington</h1>
</div>
<div id="nav">
<ul>
<li>About</li>
<li>Work</li>
<li>Contact</li>
<li>Notes</li>
</ul>
</div>
</div>
<div id="hero">
<div id="heroImage">
<img src="assets/trees.jpg" alt="trees" style="width:100%;height:10%">
</div>
<div id="overlay">
<h2>Amanda Farrington</h2>
<h3>Graphic Artist | Web Designer</h3>
View Resume
</div>
</div>
CSS
#header {
color: #D7DADB;
font-family: 'Roboto', sans-serif;
font-weight: 300;
font-size : 15px;
text-align: left;
width: 100%;
padding-left: 3em;
position: relative;
height: 15%;
box-sizing: border-box;
padding-top: 1em;
}
#header img
{
float: left;
padding-left: 3em;
}
h1{
width: 9em;
float: left;
padding-left: 0.5em;
color: #45CCCC;
padding-bottom: 1px;
}
#nav {
width: 50%;
margin:0;
padding:0;
text-align: right;
color: red;
font-size:20px;
float: right;
padding-right: 2em;
z-index: 99;
}
#nav ul {
padding: 1px;
}
#nav li {
display: inline;
padding: 38px;
}
#nav li a {
color: #2C3E50;
text-decoration: none;
}
#nav li a:hover {
color: #45CCCC;
}
/*----------hero image styles-------------*/
#hero{
padding-top: 25em;
width: 100%;
height: 30em;
position: relative;
z-index: -1;
}
#heroImage
{
top: 9%;
width: 100%;
z-index: 1;
position: absolute;
}
#overlay{
width: 34em;
top: -15%;
margin-left: 30%;
z-index: 2;
position: relative;
clear: left;
}
h2{
width: 100%;
position: relative;
font-family: 'Roboto', sans-serif;
font-weight: 300;
font-size: 60px;
float: center;
color: white;
opacity: 1.0;
text-shadow: 2px 2px 3px #000000;
text-align: center;
}
h3{
width: 100%;
position: relative;
font-family: 'Roboto', sans-serif;
font-weight: 300;
font-size: 30px;
color: #e5e5e5;
opacity: 1.0;
text-shadow: 2px 3px 2px #000000;
text-align: center;
}
a.down{
z-index: 100;
font-family: 'Roboto', sans-serif;
font-weight: 400;
text-decoration: none;
color: #181b1e;
background: #45CCCC;
position: relative;
padding: 0.6em 0.2em;
font-size: 1.2em;
-webkit-border-radius: 6px;
width: 30%;
position: relative;
display: block;
margin-left: auto;
margin-right: auto;
text-align: center;
}
a.down:hover{
text-decoration: underline;
color: white;
}

Answer 1

Because z-index works only on elements which are NOT set asposition: static. Bear in mind that every element is set as default to position:static.
Try set to position:absolute; or relative your element.
Also all other types of positioning, like position:fixed, position:sticky.

Answer 2

So I've taken a look at your code and the reason your button doesn’t work is because the div with the ID of #hero (which contains the button) is below the body because it has a z-index of -1.
Set the z-index for #hero to 0 or higher and the button will work.
#hero {
padding-top: 25em;
width: 100%;
height: 30em;
position: relative;
z-index: 0;
}
Check out this JS Fiddle I've created for you:
https://jsfiddle.net/8fqwr6ca/
Edit: Oh, and I forgot to mention–since you want the image to be below, set the #hero 's z-index to 1, set #heroImage to 0, and overlay to 2. That should do the trick (if what I think you want is correct).