CodePen

HTML

            
              <div class="rounded-inside-border">
   WTF<br>Content
</div>
            
          
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CSS

            
              .rounded-inside-border {
  height: 9em;
  width: 16em;
  background: deepskyblue;
  position: relative;
  box-sizing: border-box;
  padding: 0.5em;
  z-index: 1;
}
.rounded-inside-border:before {
  content: '';
  position: absolute;
  background: white;
  border-radius: 0.5em;
  top: 0.5em;
  bottom: 0.5em;
  left: 0.5em;
  right: 0.5em;
  z-index: -1;
}
            
          
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JS

            
              
            
          
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