Cover up text(with CSS opacity) with a solid div

Question

after researching I couldn't really find an answer so i'm asking it here. For a mobile version of my website, i'd like to have a button open up a navigation menu(which is static on the desktop version) and cover up the main portion. I wanted to have text on the main portion be visible the whole time, but when the Nav menu was open it would be faded. I found CSS's opacity function, which sounded promising Javascript worked for all of it, but the Nav div didn't block out the faded text of the main portion when it was covered. Can anyone help? Here is my code:

function openNav() {
document.getElementById("nav").style.left = "5px";
document.getElementById("list").style.opacity = "0.5";
document.getElementById("navc").style.display = "inline";
}
function closeNav() {
document.getElementById("nav").style.left = "-206px";
document.getElementById("list").style.opacity = "1";
document.getElementById("navc").style.display = "none";
}

body {
background-color: #000000;
color: #ffffff;
margin-bottom: 0;
margin-right: 5px;
margin-left: 5px;
margin-top: 5px;
}
#nav {
background-color: rgba(0, 0, 0, 1.0);
width: 188px;
margin-left: auto;
margin-right: auto;   
text-align: left;
line-height: 16px;
border: 1px solid white;
border-bottom: 0;
border-top: 0;
padding: 8px;
height: 785px;
position: absolute;
left: -206px;
overflow: auto;
}
#navm {
height: 30px;
width: 30px;
display: inline;
position: absolute;
top: 6px;
right: 6px;
}
#navc {
display: none;
height: 30px;
width: 30px;
position: absolute;
top: 6px;
right: 6px;
}
#list {
margin-left: 205px;
width: 310px;
line-height: 18px;
text-align: left;
padding: 8px;
position: absolute;
left: -208px;
}
#banner {
width: 308px;
height: 30px;
border: 1px solid white;
position: relative;
margin-top: 0;
display: block;
}

<div id="container">
<img src="/img/banner.png" alt="Banner" id="banner">
<img src="/img/navm.png" alt="" id="navm" onclick="openNav()">
<img src="/img/navc.png" alt="" id="navc" onclick="closeNav()">
<div id="nav">
<a href="/link1.html">Link 1</a><br>
<a href="/link2.html">Link 2</a><br>
<a href="/link3.html">Link 3</a><br><br>
<a href="/link4.html">Link 4</a><br><br>
</div>
<div id="list">
<h3 class="list" style="margin-top: 0">About Us</h3>
<div class="bio" style="margin-bottom: 14px">
Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget dolor. Aenean massa. Cum sociis natoque penatibus et magnis dis parturient montes, nascetur ridiculus mus. Donec quam felis, ultricies nec, pellentesque eu, pretium quis, sem. Nulla consequat massa quis enim. Donec pede justo, fringilla vel, aliquet nec, vulputate eget, arcu. In enim justo, rhoncus ut, imperdiet a, venenatis vitae, justo. Nullam dictum felis eu pede mollis pretium. Integer tincidunt. <a href="/link5.html">Link 5 &gt;&gt;</a>
</div>
  </div>

Answer 1

What you need, in my opinion is z-index. opacity only affects the visibility of current element.

Set z-index: 100 inside .nav class. That should resolve your problem.

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