Untitled

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">

    <title>CSS Question</title>

    <style type="text/css">
      .mystery,
      .mystery *
      {
        padding: 0;
        border: 0;
        margin: 0;
      }

      .button-container
      {
        background-color: #f77;
        padding: 1rem;
        margin: 1rem;
      }

      .button
      {
        float: left;
        font-family: 'Courier', monospace;
        width: 3.1rem;
        height: 2rem;
        border: solid black 2px;
        margin-right: 0.7rem;
      }

      .button-wider-1
      {
        /*
          The following 'calc' expression adds up:

            * the standard width of a button (i.e. the width
            of an element with class "button");
            * the width of a button's border;
            * the horizontal margin (i.e. the value of margin-right for a
            button);
            * the width of a button's border;
        */
        width: calc(
          3.1rem +
          2px +
          0.7rem +
          2px
        );
      }

      .button-wider-2
      {
        /*
          The following 'calc' expression is the same as the previous,
          except that there is an extra term at the end, the standard width
          of a button.
        */
        width: calc(
          3.1rem +
          2px +
          0.7rem +
          2px +
          3.1rem
        );
      }
    </style>
  </head>

  <body>
    <div class="mystery">
      <div class="button-container">
        <button type="button" class="button">x</button>
        <button type="button" class="button">x</button>
        <div style="clear: both"></div>
        <button type="button" class="button button-wider-1">y</button>
        <div style="clear: both"></div>
      </div>

      <div class="button-container">
        <button type="button" class="button">x</button>
        <button type="button" class="button">x</button>
        <div style="clear: both"></div>
        <button type="button" class="button button-wider-2">y</button>
        <div style="clear: both"></div>
      </div>
    </div>
  </body>
</html>