layout/style/test/test_pixel_lengths.html

<!DOCTYPE HTML>
<html>
<head>
  <title>Test that pixel lengths don't change based on DPI</title>
  <script type="text/javascript" src="/tests/SimpleTest/SimpleTest.js"></script>
  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css" />
</head>
<body>
<div id="display">

<div id="pt" style="width:90pt; height:90pt; background:lime;">pt</div>
<div id="pc" style="width:5pc; height:5pc; background:yellow;">pc</div>
<div id="mm" style="width:25.4mm; height:25.4mm; background:orange;">mm</div>
<div id="cm" style="width:2.54cm; height:2.54cm; background:purple;">cm</div>
<div id="in" style="width:1in; height:1in; background:magenta;">in</div>

<div id="mozmm" style="width:25.4mozmm; height:25.4mozmm; background:cyan;">mozmm</div>

</div>
<pre id="test">
<script class="testbody" type="text/javascript">

var oldDPI = SpecialPowers.getIntPref("layout.css.dpi");
var dpi = oldDPI;

function check(id, val) {
  var e = document.getElementById(id);
  is(Math.round(e.getBoundingClientRect().width), Math.round(val),
     "Checking width in " + id + " at " + dpi + " DPI");
  is(Math.round(e.getBoundingClientRect().height), Math.round(val),
     "Checking height in " + id + " at " + dpi + " DPI");
}

function checkPixelRelativeUnits() {
  check("pt", 120);
  check("pc", 80);
  check("mm", 96);
  check("cm", 96);
  check("in", 96);
}

checkPixelRelativeUnits();

SpecialPowers.setIntPref("layout.css.dpi", dpi = 96);

var mozmm = document.getElementById("mozmm");
var mozmmSize = mozmm.getBoundingClientRect().width;
is(Math.round(mozmmSize), Math.round(mozmm.getBoundingClientRect().height),
   "mozmm div should be square");

checkPixelRelativeUnits();

SpecialPowers.setIntPref("layout.css.dpi", dpi = 192);

// At 192 dpi, a one-inch box should be twice the number of device pixels,
// and since we haven't changed the device-pixels-per-CSS-pixel ratio, the
// mozmm box should be twice the size in CSS pixels.
check("mozmm", mozmmSize*2);
checkPixelRelativeUnits();

SpecialPowers.setIntPref("layout.css.dpi", oldDPI);

</script>
</pre>
</body>
</html>