<!DOCTYPE html>
<meta charset=utf-8>
<style>
@font-face {
  font-family: Fira;
  src: url(../fonts/fira/FiraSansOT-Regular.otf);
}
body {
  /* We want a font that only has a single weight, and supports 'x' and zero-width U+0301.
   * The Apple Symbols and Segoe UI Symbol fonts appear to meet this requirement on macOS
   * and Win10 respectively.
   * If they're not available, we use a single face of Fira loaded via @font-face.
   */
  font: 24px Apple Symbols, Segoe UI Symbol, Fira;
}
.bold {
  font-weight: bold;
}
span {
  color: transparent;
}
</style>
<body>
<!-- any number of zero-width characters should not result in any extra space between the 'x' glyphs ... -->
<div>
  xx<br>
  x<span>&#x301;</span>x<br>
  x<span>&#x301;&#x301;</span>x<br>
  x<span>&#x301;&#x301;&#x301;</span>x<br>
  x<span>&#x301;&#x301;&#x301;&#x301;</span>x<br>
  x<span>&#x301;&#x301;&#x301;&#x301;&#x301;</span>x<br>
  x<span>&#x301;&#x301;&#x301;&#x301;&#x301;&#x301;</span>x<br>
  x<span>&#x301;&#x301;&#x301;&#x301;&#x301;&#x301;&#x301;</span>x<br>
</div>
<!-- ... even when synthetic bold is in effect -->
<div class=bold>
  xx<br>
  x<span>&#x301;</span>x<br>
  x<span>&#x301;&#x301;</span>x<br>
  x<span>&#x301;&#x301;&#x301;</span>x<br>
  x<span>&#x301;&#x301;&#x301;&#x301;</span>x<br>
  x<span>&#x301;&#x301;&#x301;&#x301;&#x301;</span>x<br>
  x<span>&#x301;&#x301;&#x301;&#x301;&#x301;&#x301;</span>x<br>
  x<span>&#x301;&#x301;&#x301;&#x301;&#x301;&#x301;&#x301;</span>x<br>
</div>