title: CSS : Automatic width variable calculation
body: <p>Hi @all !</p>
  
  <p>let's say I have 2 helper classes as :</p>
  
  <pre><code>.w3-4 {
    width: calc((100% / 4) * 3);
  }
  
  // AND
  
  .mr-10 {
    margin-right: 10px;
  }
  </code></pre>
  
  <p>Do you think there is a automated pure css way to substract the margin value from <code>.mr-10</code> to the width from <code>.w3-4</code> ?</p>
  
  <p>Something such as </p>
  
  <pre><code>.mr-10 {
    margin-right: 10px;
  
    // width calc
    width: calc(--width - 10px);
  } 
  </code></pre>
  
  <p>There are plenty of ways to do this with SASS or even JS, but I'm just a bit lazy and curious :)</p>
  
  <p>Thanks for your answers,</p>
  
  <p>A.</p>