title: What is a fallback?
body: <p>I'm not native English speaker and I want to be sure I know what "fallback" means. I've checked that it can refer to a "Plan 'B'". More specifically, I've read this sentence from a file "For using REM and a fallback". </p>
  
  <pre><code>// Mixin: Font Size
  //
  // For using REM and a fallback
  //
  // usage:
  // @include font-size(1.2);
  //
  @mixin font-size($sizeValue: 1.6) {
    font-size: ($sizeValue * 10) + px;
    font-size: $sizeValue + rem;
  }
  </code></pre>
  
  <p>I also want to know what "fallback" means in this context. "Fallback" is it one of the two options? Are they using Fallback Pattern?</p>