CodePen

HTML

            
              <section class="left">
  Main content here
</section>

<section class="right">
  Sidebar content here
</section>
            
          
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CSS

            
              * {
 border-sizing: border-box; 
}

section.left {
  
  background:red;
  height:400px;
  
  float:left; 
  width:75%;
  margin: 0 20px 0 0; /* how do I get a 20 pixel gutter in between the main content and sidebar? */
  
}

section.right {
  
  background:blue;
  height:400px;
  
  float:right; 
  width:25%;
  
}
            
          
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JS

            
              
            
          
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