title: How to set a child's element 50px narrower to its parent element in css?
body: <p>Most tutorials just tell me something like:  </p>
  
  <pre><code>width: 100%;  
  </code></pre>
  
  <p>or something like:  </p>
  
  <pre><code>width: 200px;  
  </code></pre>
  
  <p>However, I just want to set the width of child element 50px smaller than that of the parent element. And I will place another element in the spared 50px region, so margin or padding shouldn't work for me.</p>