<!DOCTYPE html>
<!--
 | Any space immediately following another collapsible space - even one outside
 | the boundary of the inline containing that space, provided they are both
 | within the same inline formatting context - is collapsed.
 |   https://www.w3.org/TR/css3-text/#white-space-phase-1
 -->
<html>
<head>
  <style>
    body {
      margin: 0px;
      font-family: Roboto;
      font-size: 100px;
      font-weight: bold;
    }
    .not-collapsed {
      background-color: #4caf50;
    }
    .collapsed {
      background-color: #f50057;
    }
  </style>
</head>
<body>
  <div>Hello,   world!</div>
  <div>Hello,<span class="not-collapsed"> </span><span class="collapsed"> </span> world!</div>
</body>
</html>