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

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><a href="about.html">About</a></li>
        <li><a href="#workJump">Work</a></li>
        <li><a href="contact.html">Contact</a></li>
        <li><a href="notes.html">Notes</a></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>
    <a href="assets/resume.pdf" class="down">View Resume</a>                


        </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).