neovea/main.scss

## main.scss
* {
  box-sizing: border-box;
}

html, body {
  margin: 0;
}

@mixin animation() {
  transition-duration: 0.5s;
  transition-timing-function: cubic-bezier(0.65, 0.05, 0.36, 1);
}

.button-group{
  position: absolute;
  width: 65px;
  height: 65px;
  top: 0;
  left: 300px;
  margin: 8px; /* just to give some space*/
  @include animation();
}

.menu-button {
  width: 100%;
  height: 100%;
  position: absolute;
  display: block;
  background-color: #000;
  text-align: center;
  color: #fff;
  border-radius: 50%;
  left: 0;
  top: 0;
  span {
    display: block;
    width: 100%;
    height: 20px;
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%,-50%);
    @include animation();

    .fa-times {
      display: none;
    }
  }

  &.off {
    z-index: -1;
  }
}

#menu {
  position: absolute;
  height: 100vh;
  width: 300px;
  margin: 0;
  background-color: #000;
  transform: translateX(0);
  a {
    color:#fff;
    font-family: Cutive Mono;
    text-decoration: none;
  }
  @include animation();

  &:target {
    transform: translateX(-100%);
    +.button-group {
      left: 0;

      .menu-button:first-child {
        span {
          .fa-times {
            display: block;
          }
          .fa-bars {
            display: none;
          }
        }
      }

      .off {
        z-index: 0;
        background-color: transparent;
      }
    }
  }
}

## offcanvas.html
<!DOCTYPE html>
<html lang="fr">
<head>
	<meta charset="UTF-8">
	<title>Pure off canvas css menu</title>
	<link href="https://fonts.googleapis.com/css?family=Cutive+Mono" rel="stylesheet">
	<link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet">
	<link rel="stylesheet" href="stylesheets/main.css">
</head>
<body>

  <div id="menu">
    <ul>
      <li><a href="">Lorem.</a></li>
      <li><a href="">Suscipit!</a></li>
      <li><a href="">Sapiente.</a></li>
    </ul>
  </div>

  <div class="button-group">
    <a class="menu-button" href="#menu">
      <span>
      	<i class="fa fa-bars" aria-hidden="true"></i>
      	<i class="fa fa-times" aria-hidden="true"></i>
      </span>
    </a>
    <a class="menu-button off" href="#off-menu"></a>
  </div>

</body>
</html>
	* {
	box-sizing: border-box;
	}

	html, body {
	margin: 0;
	}

	@mixin animation() {
	transition-duration: 0.5s;
	transition-timing-function: cubic-bezier(0.65, 0.05, 0.36, 1);
	}

	.button-group{
	position: absolute;
	width: 65px;
	height: 65px;
	top: 0;
	left: 300px;
	margin: 8px; /* just to give some space*/
	@include animation();
	}

	.menu-button {
	width: 100%;
	height: 100%;
	position: absolute;
	display: block;
	background-color: #000;
	text-align: center;
	color: #fff;
	border-radius: 50%;
	left: 0;
	top: 0;
	span {
	display: block;
	width: 100%;
	height: 20px;
	position: absolute;
	top: 50%;
	left: 50%;
	transform: translate(-50%,-50%);
	@include animation();

	.fa-times {
	display: none;
	}
	}

	&.off {
	z-index: -1;
	}
	}

	#menu {
	position: absolute;
	height: 100vh;
	width: 300px;
	margin: 0;
	background-color: #000;
	transform: translateX(0);
	a {
	color:#fff;
	font-family: Cutive Mono;
	text-decoration: none;
	}
	@include animation();

	&:target {
	transform: translateX(-100%);
	+.button-group {
	left: 0;

	.menu-button:first-child {
	span {
	.fa-times {
	display: block;
	}
	.fa-bars {
	display: none;
	}
	}
	}

	.off {
	z-index: 0;
	background-color: transparent;
	}
	}
	}
	}
	<!DOCTYPE html>
	<html lang="fr">
	<head>
	<meta charset="UTF-8">
	<title>Pure off canvas css menu</title>
	<link href="https://fonts.googleapis.com/css?family=Cutive+Mono" rel="stylesheet">
	<link href="https://maxcdn.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet">
	<link rel="stylesheet" href="stylesheets/main.css">
	</head>
	<body>

	<div id="menu">
	<ul>
	<li><a href="">Lorem.</a></li>
	<li><a href="">Suscipit!</a></li>
	<li><a href="">Sapiente.</a></li>
	</ul>
	</div>

	<div class="button-group">
	<a class="menu-button" href="#menu">
	<span>
	<i class="fa fa-bars" aria-hidden="true"></i>
	<i class="fa fa-times" aria-hidden="true"></i>
	</span>
	</a>
	<a class="menu-button off" href="#off-menu"></a>
	</div>

	</body>
	</html>