https://medium.com/@heyoka/responsive-pure-css-off-canvas-hamburger-menu-aebc8d11d793

Also viewable in GitHub Repo: https://gist.github.com/markcaron

---------------------------------------------------Original Code from Source-----------------------------------------------------------------------

---------------------------------------HTML-------------------------------------------------------

<header>
  <a href="#main-menu"
     id="main-menu-toggle"
     class="menu-toggle"
     aria-label="Open main menu">
    <span class="sr-only">Open main menu</span>
    <span class="fa fa-bars" aria-hidden="true"></span>
  </a>
  
  <h1 class="logo">hamburgers</h1>
  
  <nav id="main-menu" class="main-menu" aria-label="Main menu">
    <a href="#main-menu-toggle"
       id="main-menu-close"
       class="menu-close"
       aria-label="Close main menu">
      <span class="sr-only">Close main menu</span>
      <span class="fa fa-close" aria-hidden="true"></span>
    </a>
    <ul>
      <li><a href="#">Products</a></li>
      <li><a href="#">About</a></li>
      <li><a href="#">Contact</a></li>
    </ul>
  </nav>
  <a href="#main-menu-toggle"
     class="backdrop"
     tabindex="-1"
     aria-hidden="true" hidden></a>
</header>

---------------------------------------CSS-------------------------------------------------------

/* Screen reader only */
.sr-only {
  position: absolute;
  width: 1px;
  height: 1px;
  padding: 0;
  margin: -1px;
  overflow: hidden;
  clip: rect(0,0,0,0);
  border: 0;
}

/* Button styling */
.menu-toggle {
  display: inline-block;
  padding: .75em 15px;
  line-height: 1em;
  font-size: 1em;
  color: #333;
}

.menu-toggle:hover,
.menu-toggle:focus {
  color: #c00;
}

/*
 Default styles + Mobile first
 Offscreen menu style
*/
.main-menu {
  position: absolute;
  display: none;
  left: -200px;
  top: 0;
  height: 100%;
  overflow-y: scroll;
  overflow-x: visible;
  transition: left 0.3s ease,
    	box-shadow 0.3s ease;
  z-index: 999;
}

.main-menu ul {
  list-style: none;
  margin: 0;
  padding: 2.5em 0 0;
  /* Hide shadow w/ -8px while 'closed' */
  -webkit-box-shadow: -8px 0 8px rgba(0,0,0,.5);
     -moz-box-shadow: -8px 0 8px rgba(0,0,0,.5);
          box-shadow: -8px 0 8px rgba(0,0,0,.5);
  min-height: 100%;
  width: 200px;
  background: #1a1a1a;
}

.main-menu a {
  display: block;
  padding: .75em 15px;
  line-height: 1em;
  font-size: 1em;
  color: #fff;
  text-decoration: none;
  border-bottom: 1px solid #383838;
}

.main-menu li:first-child a {
  border-top: 1px solid #383838;
}

.main-menu a:hover,
.main-menu a:focus {
  background: #333;
  text-decoration: underline;
}

.main-menu .menu-close {
  position: absolute;
  right: 0;
  top: 0;
}


/*
 More to come here...
*/


/*
 Demo purposes only
*/
*,
*:before,
*:after {
  box-sizing: border-box;
}

header {
  padding: 20px;
  display: flex;
  align-items: baseline;
}

article {
  padding: 30px;
  width: 55em;
  font-size: 16px;
  line-height: 1.5em;
}

article h2 {
  font-weight: 500;
  font-size: 28px;
}

.logo {
  margin: 0 30px 0 10px;
  font-size: 1.5em;
}

/*
 On small devices, allow it to toggle...
*/
/*
 :target for non-JavaScript
 [aria-expanded] will be used if/when JavaScript is added to improve interaction, though it's completely optional.
*/
.main-menu:target,
.main-menu[aria-expanded="true"] {
  display: block;
  left: 0;
  outline: none;
  -moz-box-shadow: 3px 0 12px rgba(0,0,0,.25);
  -webkit-box-shadow: 3px 0 12px rgba(0,0,0,.25);
  box-shadow: 3px 0 12px rgba(0,0,0,.25);
}

.main-menu:target .menu-close,
.main-menu[aria-expanded="true"] .menu-close {
  z-index: 1001;
}

.main-menu:target ul,
.main-menu[aria-expanded="true"] ul {
  position: relative;
  z-index: 1000;
}

/* 
 We could us `.main-menu:target:after`, but
 it wouldn't be clickable.
*/
.main-menu:target + .backdrop,
.main-menu[aria-expanded="true"] + .backdrop{
  position: absolute;
  display: block;  
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  z-index: 998;
  background: #000;
  background: rgba(0,0,0,.85);
  cursor: default;
}

@supports (position: fixed) {
  .main-menu,
  .main-menu:target + .backdrop,
  .main-menu[aria-expanded="true"] + .backdrop{
    position: fixed;
  }
}

-------------------------------------------------------------- Edited code for site-------------------------------------------------------------------------------------

---------------------------------------HTML-------------------------------------------------------

<nav id="main-menu" class="main-menu" aria-label="Main menu">
<a href="#main-menu-toggle" id="main-menu-close" class="menu-close" aria-label="Close main menu">
    <span class="sr-only">Close main menu</span>
    <span class="fa fa-close" aria-hidden="true"></span>
</a>
<ul>
    <li><a href="index.html" class="list-active">Home</a></li>
    <li><a href="islands.html">Islands</a></li>
    <li><a href="types.html">Types</a></li>
    <li><a href="gallery.html">Gallery</a></li>
    <li><a href="contact.html">Contact Us</a></li>
</ul>
</nav>
<a href="#main-menu-toggle" class="backdrop" tabindex="-1" aria-hidden="true" hidden></a>
<a href="#main-menu" id="main-menu-toggle" class="menu-toggle" aria-label="Open main menu">
    <span class="fa fa-bars" aria-hidden="true"></span>
</a>

---------------------------------------CSS-------------------------------------------------------

.menu-toggle {
    display: inline-block;
    padding: 15px;
    }
.main-menu {
    position: absolute;
    display: none;
    left: -200px;
    top: 0;
    height: 240px;
    transition: left 0.3s ease,
          box-shadow 0.3s ease;
    z-index: 999;

    }
.main-menu ul {
    list-style: none;
    margin: 0;
    padding: 2.5em 0 0;
    height: 240px;
    width: 200px;
    background: #1a1a1a;
    }
.main-menu a {
    display: block;
    padding: .75em 15px;
    line-height: 1em;
    font-size: 1em;
    color: #fff;
    text-decoration: none;
    border-bottom: 1px solid #383838;
    }
.small-link{
    font-size: 0.7em;
}    
.main-menu li:first-child a {
    border-top: 1px solid #383838;
    }
.main-menu a:hover, .main-menu a:focus {
    background: #333;
    text-decoration: underline;
    }
.main-menu .menu-close {
    position: absolute;
    right: 0;
    top: 0;
    }

/* Inside the pop-out menu --------------------------------- */
.main-menu:target,
.main-menu[aria-expanded="true"] {
  display: block;
  left: 0;
  outline: none;
}

.main-menu:target .menu-close,
.main-menu[aria-expanded="true"] .menu-close {
  z-index: 1001;
}

.main-menu:target ul,
.main-menu[aria-expanded="true"] ul {
  position: relative;
  z-index: 1000;
}