CodePen

HTML

            
              
            
          
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CSS

            
              html {
  border: 200px solid white;
  padding-bottom: 30%;
  background: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVQIW2NkYPj/HwADBwIAcbCYagAAAABJRU5ErkJggg==);
}
            
          
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JavaScript

            
              
            
          
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999px
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