<!-- Optional content you want to persist between sections -->
<div class="content">BLEND</div>

<section class="one">
  <div class="wrapper"></div>
</section>

<section class="two">
  <div class="wrapper"></div>
</section>

<section class="three">
  <div class="wrapper"></div>
</section>

<section class="four">
  <div class="wrapper"></div>
</section>
%fixedbox {
  // Basic styling
  box-sizing: border-box;
  display: block;
  font-size: 4em;
  padding: 1em;
  width: 6em;
  height: 3em;
  
  // Center box in middle of screen
  position: fixed; 
    top: 50%;
    left: 50%;
  transform: translate3d(-50%,-50%,0);
}

.content {
    // Basic styling
    @extend %fixedbox;
  
    font-weight: bold;
    text-align: center;
    z-index: 1000;
  
    // Cut-out text effect
    background: #fff;
    color: #000;
    mix-blend-mode: lighten;
}

section {
  // Background image styling
  background-repeat: no-repeat;
  background-size: cover;
  
  // Reset stacking context
  isolation: isolate;
  
  // Must be set to correctly position .wrapper below
  position: relative;
  
  // Height not required. Can be left as auto
  height: 100vh;
  padding: 1em;
}

.wrapper {
  // Hides box (&:before) outside of section
  clip: rect(auto auto auto auto);
  
  // Clip works only on positions absolute or fixed
  position: absolute; 
  // Stretch to fill entire section
    top: 0;
    right: 0;
    bottom: 0;
    left: 0;
  
  &:before {
    content: " ";
    
    // Basic styling
    @extend %fixedbox;
    padding: 2em 3.5em;
    
    // Blend with background
    mix-blend-mode: darken;

  }
}

.one {
  // Section background image
  background-image: url(https://unsplash.it/1920/1080?image=737);
  
  .wrapper:before {
    // Box background color
    background: red;
  }
}

.two {
  background-image: url(https://unsplash.it/1920/1080?image=812);
  
  .wrapper:before {
    background: #faaa54;
    mix-blend-mode: difference;
  }
}

.three {
  background-image: url(https://unsplash.it/1920/1080?image=810);
  
  .wrapper:before {
    background: #6c320a;
    mix-blend-mode: screen;
  }
}

.four {
  background-image: url(https://unsplash.it/1920/1080?image=634);
  
  .wrapper:before {
    background: #e4135d;
  }
}
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