title: Transparent Border showing background image
body: <p>I have this:</p>
  
  <p><img src="https://i.stack.imgur.com/6ovwK.jpg" alt="Solid border that should be transparent"></p>
  
  <p>I want to achieve this:</p>
  
  <p><img src="https://i.stack.imgur.com/rRQzU.jpg" alt="Transparent border showing background image"></p>
  
  <p>I have a big outer div (with red background) and a smaller-inner div (with green background). The small div has a border, I want the border to appear as transparent to show the behind background. Is this achievable with HTML/CSS?</p>